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Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

I had some thoughts about precise calculation of drift of the SDEs for simulation in Ito-hermite framework. Proper calculation of drift is not a huge problem when we are dealing with monte carlo simulations but in Ito-hermite method of moving and expanding/contracting grid, any inexactness in proper calculation of drift leads to loss of accuracy. I thought of using Girsanov's theorem for proper calculation of drift of the SDEs. This is slightly different from the use of Girsanov for change of drift usually employed in simulations in Finance.
Let us consider a very simple example of a very basic SDE with constant drift and constant volatility. Here is the SDE

$dX(t)=\mu dt + \sigma dz(t)$,$X(0)=x$
Suppose $P$ is the probability density/measure  associated with the above SDE at time t. Let us consider another SDE
$dY(t)=\sigma dz(t)$,$Y(0)=x$
We further Suppose $Q$ is the probability density/measure associated with the second SDE without drift at time t.
Under some regular assumptions, we know that change of probabilty Radon-Nikodym derivative between two densities/measures at time t is given as
$M_t=\exp[\int_0^t \frac{\mu}{\sigma} dz(s) - .5 \int_0^t {(\frac{\mu}{\sigma})}^2 ds]$
or $\frac{dP}{dQ}=M_t$
or in other words
$\int f(X) dP =\int f(X) \frac{dP}{dQ} dQ$

We can in fact construct the density of SDE with drift after Multiplying the Girsanov exponential with the density of simpler driftless SDE.
If we look at the Girsanov exponential, it is closely related to exponential generating function of hermite polynomials and it can be expanded just like the hermite exponential generating function as
$\exp(z(t) -.5 t )=\sum_{n=0}^{\infty} \frac{H_n(t)}{n!}$
where $H_n(t)$ are brownian motion hermite polynomials of nth degree.
Once we expand the Girsanov exponential associated with our constant drift SDE, we get
$M_t=\exp[\int_0^t \frac{\mu}{\sigma} dz(s) - .5 \int_0^t {(\frac{\mu}{\sigma})}^2 ds]=\sum_{n=0}^{\infty} {(\frac{\mu}{\sigma})}^n \frac{H_n(t)}{n!}$
In context of Ito-hermite framework, Please note that for a constant drift SDE where drift makes an equal contribution along every grid subdivision, the coefficients of hermite polynomials in the above equation are constant along the grid.
Let us consider  a general SDE with two variable drift terms and a simple driving brownian motion
$dX(t)=\mu_1 X^{\beta_1} dt + \mu_2 X^{\beta_2} dt + \sigma dz(t)$
In order to convert the above SDE into an SDE without drift(i.e into simple scaled brownian motion as in our case here), the change of probability Radon-Nikodym derivative that will convert the above SDE with variable drift into another simple brownian noise SDE will be given by Girsanov exponential as

$M_t=\exp[-\int_0^t \frac{(\mu_1 X^{\beta_1} + \mu_2 X^{\beta_2})}{\sigma} dz(s) - .5 \int_0^t {(\frac{(\mu_1 X^{\beta_1} + \mu_2 X^{\beta_2})}{\sigma})}^2 ds]$
In our Ito-hermite framework, since we have a very small time step, the above exponentials would be re-calculated starting from time zero for every interval and should not be extremely difficult to evaluate.
The above exponential can also be decomposed using exponential generating function into hermite polynomials though we will have to make more detailed calculations for that purpose. We can write
$M_t=\exp[-\int_0^t \frac{(\mu_1 X^{\beta_1} + \mu_2 X^{\beta_2})}{\sigma} dz(s) - .5 \int_0^t {(\frac{(\mu_1 X^{\beta_1} + \mu_2 X^{\beta_2})}{\sigma})}^2 ds]$
$=\sum_{n=0}^{\infty} C_n \frac{H_n(t)}{n!}$

Where we have to obtain the coefficients $C_n$ in above equation after detailed calculations. In general, for a variable drift SDE, these coefficients will be quite different for each grid subdivision in our Ito-hermite framework. In our first basic example of constant drift SDE, $C_n={(\frac{\mu}{\sigma})}^n$ but in constant drift case, these coefficients are constant along the grid with $\frac{\partial C_n}{\partial Z}=0$
I believe it is the derivatives of the above coefficients(with respect to Z associated with each subdivision) for each hermite polynomial $C_n$ along the grid that determine how drift changes along the Ito-hermite expanding/contracting grid subdivisions. We can represent the above mentioned derivatives of coefficients of nth hermite polynomial symbolically as $\frac{\partial C_n}{\partial Z}$ which are calculated along each subdivision in our Ito-hermite grid.  We can find the drift at $Z=0$ in our Ito-hermite grid and then reconstruct the drift in other grid points using the derivatives of the hermite polynomials in the expansion of the Girsanov exponential. I believe this will be very slightly different and possibly more precise as compared to the direct calculation of drift we have pursued in our Ito-hermite framework and in monte carlo simulations.
I will come back to experiments with these ideas after I have completed the program for the calculation of simulation coefficients for explicitly time dependent SDEs. Here is the progress on the explicit time dependent SDEs project. As I had mentioned, I have chosen the path of representation of time functions in the form of their Taylor series. Until now I have been dealing with taylor series of exponential function. I have written programs for calculation of variance and other stocahstic integrals. After calculation of variance, I re-convert the variance series back into "volatility series" by taking the square-root of the power series. Square root of a power series usually does not exist if the first few terms of the power series are zero which is mostly the case for the variance series but I just take some required powers of t common out of the series so that leading term is non-zero positive and then we can take the square-root of the variance series to convert it into a volatility series and use it again for higher order calculations of stochastic integrals and this works very well for the exponential functions in the drift and volatility. I hope to complete the program in a few days and then will come back to experiments with Ito-hermite method.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

I had some thoughts about precise calculation of drift of the SDEs for simulation in Ito-hermite framework. Proper calculation of drift is not a huge problem when we are dealing with monte carlo simulations but in Ito-hermite method of moving and expanding/contracting grid, any inexactness in proper calculation of drift leads to loss of accuracy. I thought of using Girsanov's theorem for proper calculation of drift of the SDEs. This is slightly different from the use of Girsanov for change of drift usually employed in simulations in Finance.
Let us consider a very simple example of a very basic SDE with constant drift and constant volatility. Here is the SDE

$dX(t)=\mu dt + \sigma dz(t)$,$X(0)=x$
Suppose $P$ is the probability density/measure  associated with the above SDE at time t. Let us consider another SDE
$dY(t)=\sigma dz(t)$,$Y(0)=x$
We further Suppose $Q$ is the probability density/measure associated with the second SDE without drift at time t.
Under some regular assumptions, we know that change of probabilty Radon-Nikodym derivative between two densities/measures at time t is given as
$M_t=\exp[\int_0^t \frac{\mu}{\sigma} dz(s) - .5 \int_0^t {(\frac{\mu}{\sigma})}^2 ds]$
or $\frac{dP}{dQ}=M_t$
or in other words
$\int f(X) dP =\int f(X) \frac{dP}{dQ} dQ$

We can in fact construct the density of SDE with drift after Multiplying the Girsanov exponential with the density of simpler driftless SDE.
If we look at the Girsanov exponential, it is closely related to exponential generating function of hermite polynomials and it can be expanded just like the hermite exponential generating function as
$\exp(z(t) -.5 t )=\sum_{n=0}^{\infty} \frac{H_n(t)}{n!}$
where $H_n(t)$ are brownian motion hermite polynomials of nth degree.
Once we expand the Girsanov exponential associated with our constant drift SDE, we get
$M_t=\exp[\int_0^t \frac{\mu}{\sigma} dz(s) - .5 \int_0^t {(\frac{\mu}{\sigma})}^2 ds]=\sum_{n=0}^{\infty} {(\frac{\mu}{\sigma})}^n \frac{H_n(t)}{n!}$
In context of Ito-hermite framework, Please note that for a constant drift SDE where drift makes an equal contribution along every grid subdivision, the coefficients of hermite polynomials in the above equation are constant along the grid.
Let us consider  a general SDE with two variable drift terms and a simple driving brownian motion
$dX(t)=\mu_1 X^{\beta_1} dt + \mu_2 X^{\beta_2} dt + \sigma dz(t)$
In order to convert the above SDE into an SDE without drift(i.e into simple scaled brownian motion as in our case here), the change of probability Radon-Nikodym derivative that will convert the above SDE with variable drift into another simple brownian noise SDE will be given by Girsanov exponential as

$M_t=\exp[-\int_0^t \frac{(\mu_1 X^{\beta_1} + \mu_2 X^{\beta_2})}{\sigma} dz(s) - .5 \int_0^t {(\frac{(\mu_1 X^{\beta_1} + \mu_2 X^{\beta_2})}{\sigma})}^2 ds]$
In our Ito-hermite framework, since we have a very small time step, the above exponentials would be re-calculated starting from time zero for every interval and should not be extremely difficult to evaluate.
The above exponential can also be decomposed using exponential generating function into hermite polynomials though we will have to make more detailed calculations for that purpose. We can write
$M_t=\exp[-\int_0^t \frac{(\mu_1 X^{\beta_1} + \mu_2 X^{\beta_2})}{\sigma} dz(s) - .5 \int_0^t {(\frac{(\mu_1 X^{\beta_1} + \mu_2 X^{\beta_2})}{\sigma})}^2 ds]$
$=\sum_{n=0}^{\infty} C_n \frac{H_n(t)}{n!}$

Where we have to obtain the coefficients $C_n$ in above equation after detailed calculations. In general, for a variable drift SDE, these coefficients will be quite different for each grid subdivision in our Ito-hermite framework. In our first basic example of constant drift SDE, $C_n={(\frac{\mu}{\sigma})}^n$ but in constant drift case, these coefficients are constant along the grid with $\frac{\partial C_n}{\partial Z}=0$
I believe it is the derivatives of the above coefficients(with respect to Z associated with each subdivision) for each hermite polynomial $C_n$ along the grid that determine how drift changes along the Ito-hermite expanding/contracting grid subdivisions. We can represent the above mentioned derivatives of coefficients of nth hermite polynomial symbolically as $\frac{\partial C_n}{\partial Z}$ which are calculated along each subdivision in our Ito-hermite grid.  We can find the drift at $Z=0$ in our Ito-hermite grid and then reconstruct the drift in other grid points using the derivatives of the hermite polynomials in the expansion of the Girsanov exponential. I believe this will be very slightly different and possibly more precise as compared to the direct calculation of drift we have pursued in our Ito-hermite framework and in monte carlo simulations.
I will come back to experiments with these ideas after I have completed the program for the calculation of simulation coefficients for explicitly time dependent SDEs. Here is the progress on the explicit time dependent SDEs project. As I had mentioned, I have chosen the path of representation of time functions in the form of their Taylor series. Until now I have been dealing with taylor series of exponential function. I have written programs for calculation of variance and other stocahstic integrals. After calculation of variance, I re-convert the variance series back into "volatility series" by taking the square-root of the power series. Square root of a power series usually does not exist if the first few terms of the power series are zero which is mostly the case for the variance series but I just take some required powers of t common out of the series so that leading term is non-zero positive and then we can take the square-root of the variance series to convert it into a volatility series and use it again for higher order calculations of stochastic integrals and this works very well for the exponential functions in the drift and volatility. I hope to complete the program in a few days and then will come back to experiments with Ito-hermite method.
Borrowing from the above message, I quote the lines below
[Quote]Once we expand the Girsanov exponential associated with our constant drift SDE, we get
$M_t=\exp[\int_0^t \frac{\mu}{\sigma} dz(s) - .5 \int_0^t {(\frac{\mu}{\sigma})}^2 ds]=\sum_{n=0}^{\infty} {(\frac{\mu}{\sigma})}^n \frac{H_n(t)}{n!}$ [Unquote]
As we see that the Girsanove exponential in the constant drift SDE has coefficients of hermite polynomials (I absorbed sqrt(t) and its powers in the hermite polynomials and called them Brownain motion hermite polynomials but powers of sqrt(t)  also has to be considered multiples of coefficients and we have to use standard normal hermite polynomials) which are simple powers of a constant expression. We can think of these coefficients on standard normal hermite polynomials as local moments. In our constant drift case, all local moments are powers of each other and hence there is just first local cumulant and all higher local cumulants are zero. we have to convert above local moments of Girsanov drift exponential into local cumulants and then add then properly calculate the coefficients of hermite polynomials that have to be added to the diffusion in order to add the effect of drift in the existing distribution. In our constant drift case, there is no second cumulant in the Girsanov exponential so it does not effect the variance and only adds a constant drift to the SDE.
So again we can calculate the Girsanov exponential for the drift of an SDE with several drift terms especially when our diffusion is scaled normal. And then we can expand the Girsanov exponential into hermite polynomials and its coefficietns (which are local moments of the drift distribution) and then we can convert these local moments into cumulants and properly calculate the coefficients of the hermite polynoamials that have to be added to the existing distribution after accounting for the effect of linear diffusion as in our Ito-hermite framework.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

Here is a link to a very good paper explaining some ideas in my post in more detail in context of moments, cumulants and hermite representation. Link to the paper is here: http://web.ist.utl.pt/~berberan/data/115.pdf
And this would also be helpful.
https://en.wikipedia.org/wiki/Edgeworth_series

The above links are helpful but I would myself want to try to do something on the lines of converting "local moments" into "local cumulants" and then into a "local hermite representation". By local moments and local representation I mean something that would be different for each subdivision in out Ito-hermite framework.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

I did some work on Ito-hermite program and now it works very well with mean correction. I have still not made the change for mean reverting SDEs that cancel a term but introduce time dependent coefficients with time exponentials but the program is extremely good when mean is known. When the mean of the SDE is known, program is almost absolutely perfect. You can very easily use the program to work with general two-drift terms SDEs but the current command-line version works with mean reversion type equations and you have to specify kappa, theta, vol and terminal time. But you can easily uncomment the general program structure with two drift terms. The program will work extremely well when mean is analytically known and you will have to specify the mean.
Here is the code for the Ito-hermite program. I have also posted code for the new functions ( actually they are the same as I posted when I posted Ito-hermite program last time) but I am not posting the old monte carlo simulations coefficient calculation programs as I have previously posted them many times.
function [TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected04(x0,theta,kappa,gamma,sigma0,T)

%Copyright Ahsan Amin. Infiniti derivatives Technologies.
%or skype ahsan.amin2999
%In this program, I am simulating the SDE given as
%dy(t)=mu1 x(t)^beta1 dt + mu2 x(t)^beta2 dt +sigma x(t)^gamma dz(t)

%I have not directly simulated the SDE but simulated the transformed
%Besse1l process version of the SDE and then changed coordinates to retreive
%the SDE in original coo
%rdinates.
%The present program will analytically evolve only the Bessel Process version of the
%SDE in transformed coordinates.
%In this version, the mean correction has been disabled. If you are
%simulating standard mean-reverting SV type SDEs, please enable the mean
%correction by uncommenting the appropriate line in the body of code below.

dt=.125/16;   % Simulation time interval.%Fodiffusions close to zero
%decrease dt for accuracy.
%T=5;
Tt=128*T;%16*16;     % Number of simulation levels. Terminal time= Tt*dt; //.125/32*32*16=2 year;
OrderA=4;  %
OrderM=4;  %
%dtM=.125/8;%Monte carlo time interval size dtM.
%TtM=8*8;%Monte carlo number of simulation intervals.
dtM=dt*2;
TtM=Tt/2;

dNn=.2/1;   % Normal density subdivisions width. would change with number of subdivisions
Nn=48;  % No of normal density subdivisions
NnMidl=25;%One half density Subdivision left from mid of normal density(low)
NnMidh=26;%One half density subdivision right from the mid of normal density(high)
NnMid=4.0;

%theta=mu1/(-mu2);
%kappa=-mu2;

%x0=.25;   % starting value of SDE
beta1=0.0;
beta2=1.0;   % Second drift term power.
%gamma=.95;%50;   % volatility power.
%kappa=.5;   %mean reversion parameter.
%theta=.075;%mean reversion target

%you can specify any general mu1 and mu2 and beta1 and beta2.
%The present program is not perfect but quite reasonably good.
%if there is only one term in drift with beta=0, there will be minor
%errors. This case is yet not covered.

mu1=+1*theta*kappa;   %first drift coefficient.
mu2=-1*kappa;    % Second drift coefficient.
%sigma0=1.0;%Volatility value
alpha=1;% x^alpha is being expanded. This is currently for monte carlo only.
alpha1=1-gamma;

w(1:Nn)=x0^(1-gamma)/(1-gamma);

Z(1:Nn)=(((1:Nn)-5.5)*dNn-NnMid);

Z
str=input('Look at Z');
ZProb(1)=normcdf(.5*Z(1)+.5*Z(2),0,1)-normcdf(.5*Z(1)+.5*Z(2)-dNn,0,1);
ZProb(Nn)=normcdf(.5*Z(Nn)+.5*Z(Nn-1)+dNn,0,1)-normcdf(.5*Z(Nn)+.5*Z(Nn-1),0,1);

for nn=2:Nn-1
ZProb(nn)=normcdf(.5*Z(nn)+.5*Z(nn+1),0,1)-normcdf(.5*Z(nn)+.5*Z(nn-1),0,1);
%Above calculate probability mass in each probability subdivision.
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

sigma11(1:OrderA+1)=0;
mu11(1:OrderA+1)=0;
mu22(1:OrderA+1)=0;
sigma22(1:OrderA+1)=0;
% index 1 correponds to zero level since matlab indexing starts at one.
sigma11(1)=1;
mu11(1)=1;
mu22(1)=1;
sigma22(1)=1;

for k=1:(OrderA+1)
if sigma0~=0
sigma11(k)=sigma0^(k-1);
end
if mu1 ~= 0
mu11(k)=mu1^(k-1);
end
if mu2 ~= 0
mu22(k)=mu2^(k-1);
end
if sigma0~=0
sigma22(k)=sigma0^(2*(k-1));
end
end
%Ft(1:TtM+1,1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0; %General time powers on hermite polynomials
Fp(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers on coefficients of hermite polynomials.
Fp1(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers for bessel transformed coordinates.

%YCoeff0 and YCoeff are coefficents for original coordinates monte carlo.
%YqCoeff0 and YqCoeff are bessel/lamperti version monte carlo.

YCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
YqCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
%Pre-compute the time and power exponent values in small multi-dimensional arrays
YCoeff = ItoTaylorCoeffsNew(alpha,beta1,beta2,gamma); %expand y^alpha where alpha=1;
YqCoeff = ItoTaylorCoeffsNew(alpha1,beta1,beta2,gamma);%expand y^alpha1 where alpha1=(1-gamma)
YqCoeff=YqCoeff/(1-gamma); %Transformed coordinates coefficients have to be
%further divided by (1-gamma)

for k = 0 : (OrderA)
for m = 0:k
l4 = k - m + 1;
for n = 0 : m
l3 = m - n + 1;
for j = 0:n
l2 = n - j + 1;
l1 = j + 1;
%Ft(l1,l2,l3,l4) = dtM^((l1-1) + (l2-1) + (l3-1) + .5* (l4-1));
Fp(l1,l2,l3,l4) = (alpha + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));
Fp1(l1,l2,l3,l4) = (alpha1 + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));

YCoeff0(l1,l2,l3,l4) =YCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
YqCoeff0(l1,l2,l3,l4) =YqCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
end
end
end
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
wnStart=1;%
d2wdZ2(1:Nn)=0;
dwdZ(1:Nn)=0;
yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
tic

for tt=1:Tt

yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

[wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,c1,c2,c3,c4] = CalculateDriftAndVolOrder4(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt);

dw(wnStart:Nn)=c1(wnStart:Nn).*Z(wnStart:Nn) ;% ...
dw2(wnStart:Nn)=dw(wnStart:Nn).^2;
% d2w(wnStart:Nn)=c2(wnStart:Nn).*(Z(wnStart:Nn).^2-1);
% d3w(wnStart:Nn)=c3(wnStart:Nn).*(Z(wnStart:Nn).^3-3*Z(wnStart:Nn));
w(isnan(w)==1)=0;
wMu0dt(isnan(wMu0dt)==1)=0;
wMeanPrev=sum(ZProb(wnStart:Nn).*w(wnStart:Nn));%Calculate the mean.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

[w0,Sigma00,Sigma11,Sigma22,Sigma33,Sigma1,Sigma2,Sigma3,Sigma0] = CalculateHermiteRepresentationOfDensityOrderThreeNew(w,Z,wnStart,Nn,dNn,NnMidh,NnMidl);
d2wdZ2(wnStart:Nn)=d2wdZ2(wnStart:Nn)+ d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2+dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn);%+ ...
dwdZ(wnStart:Nn)=(Sigma00)+Sigma1(wnStart:Nn).*2.*Z(wnStart:Nn);

H2(wnStart:Nn)=(Z(wnStart:Nn).^2-1);
CTerm(wnStart:Nn)=(.5*d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2 - ...
1*dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn));
Correction1(wnStart:Nn)=-sqrt(2/3)/(22/7)^2.*CTerm(wnStart:Nn).*H2(wnStart:Nn);
Correction0(wnStart:Nn)=-sqrt(2/3)/(22/7)^2.*(d2wdZ2(wnStart:Nn)-.25*dwdZ(wnStart:Nn).^2).*H2(wnStart:Nn);

if(tt==1)
w(wnStart:Nn)=wMeanPrev+wMu0dt(wnStart:Nn)+ ...
sign(w(wnStart:Nn)-wMeanPrev+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn)));
end
if(tt>1)

w(wnStart:Nn)=wMeanPrev+wMu0dt(wnStart:Nn)+ ...
sign(w(wnStart:Nn)-wMeanPrev+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn)))+ ...
+dt *Correction0(wnStart:Nn) + ...
Correction1(wnStart:Nn);

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Below are instability correction equations. There is instability in the
% SDE representation equation roughly between -.5 SD to +.5 SD in normal terms.
%Instability correction Algorithm takes two ends that are here given as
%lower end=w(NnMidl-2) and upper end=w(NnMidh+3).
%In the block below the interpolation starts from lower end.
%1st equation is
%And so on.
%There is a sixth equation that is not written below(since it takes us to
%the upper interpolation point which is already known) which is
%Sum of these six equations add up to the distance between upper and lower
%interpolation points.
%In the sum there are six instances of first derivative which is already
%calculated at the boundary, six instances of second derivative which is also
%already calculated at lower boundary and 18.0 instances of DeltaD1w which is
%unknown. This equation calculates the value of one unit of DeltaD1w addition
% I hope to find a better analytical method to tackle this issue. Friends
% are welcome to play with slightly changed related alternatives to the
%above interpolation.
CTheta=1.0;
D1wl=(11/6*w(NnMidl-2)-3*w(NnMidl-3)+3/2*w(NnMidl-4)-1/3*w(NnMidl-5))/dNn;
D2wl=(35/12*w(NnMidl-2) -26/3*w(NnMidl-3)+ 19/2*w(NnMidl-4)-14/3*w(NnMidl-5)+11/12*w(NnMidl-6))/dNn.^2;

%The last equation is not used since point w(NnMidh+3)is given but it is used in
%calculation of distance between two interpolation points. This distance is
%used to determine the value of DeltaD1w.

D1wh=(-11/6*w(NnMidh+3)+3*w(NnMidh+4)-3/2*w(NnMidh+5)+1/3*w(NnMidh+6))/dNn;
D2wh=(35/12*w(NnMidh+3) -26/3*w(NnMidh+4)+ 19/2*w(NnMidh+5)-14/3*w(NnMidh+6)+11/12*w(NnMidh+7))/dNn.^2;

%
w(NnMidh+2)=.5*whh(NnMidh+2)+.5*wll(NnMidh+2);
w(NnMidh+1)=.5*whh(NnMidh+1)+.5*wll(NnMidh+1);
w(NnMidh)=.5*whh(NnMidh)+.5*wll(NnMidh);
w(NnMidl)=.5*whh(NnMidl)+.5*wll(NnMidl);
w(NnMidl-1)=.5*whh(NnMidl-1)+.5*wll(NnMidl-1);

%       w1(1:Nn-1)=w(1:Nn-1);
%       w2(1:Nn-1)=w(2:Nn);

w1(1:Nn-2)=w(1:Nn-2);
w2(1:Nn-2)=w(2:Nn-1);
w3(1:Nn-2)=w(3:Nn);
w(w1(:)>w2(:))=w2(w1(:)>w2(:))-.25*(w3(w1(:)>w2(:))-w2(w1(:)>w2(:)));
%      w(w1(:)>w2(:))=0;%Be careful;might not universally hold;

w(w<0)=0.0;
for nn=1:Nn
if(w(nn)<=0)
wnStart=nn+1;
end
end

%%1st order mean correction. We know the mean in original
%%coordinates so I shift the density into original coordinates,
%%apply the mean correction and then transform again into Lamperti
%%coordinates. Algebra solves two equations in two unknowns.
%%Equations are Sum_0^N{(Y_w(wnStart:Nn)-Y0)*W0}=1 and the second
%%equation is Sum_0^N{Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0}=u0
%%Two unknows are Y0 and W0. u0 is known mean.
tt;
u0=theta+(x0-theta)*exp(-kappa*(tt*dt)); %analytic mean of the density

%If you are not using stochastic volatility, replace above with
%true mean otherwise results would become garbage.

Y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

Yn2Pn=sum(Y_w(wnStart:Nn).*Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
YnPn=sum(Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
Pn=1.0;%%Sum(ZProb(1:Nn))
Y0=(Yn2Pn-u0*YnPn)/(YnPn-u0*Pn);

Y0Pn=Y0*Pn;
W0=1/(YnPn-Y0Pn);
YCorrected(wnStart:Nn)=Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0;
wCorrected(wnStart:Nn)=(YCorrected(wnStart:Nn).^(1-gamma))./(1-gamma);
w(wnStart:Nn)=wCorrected(wnStart:Nn);

%I have disabled the mean correction. The above mean correction is only valid for
%standard mean reverting stochastic volatility type SDEs. To enable mean
%correction please uncomment the above last line in the block.

end

y_w(1:Nn)=0;
y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
Dfy_w(wnStart:Nn)=0;
Dfw(wnStart:Nn)=0;
for nn=wnStart+1:Nn-1
Dfy_w(nn) = (y_w(nn + 1) - y_w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
Dfw(nn) = (w(nn + 1) - w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
%Change of variable derivative for densities
end
fy_w(1:Nn)=0;
fw(1:Nn)=0;
for nn = wnStart:Nn-1
fy_w(nn) = (normpdf(Z(nn),0, 1))/abs(Dfy_w(nn));%Origianl coordinates density
fw(nn) = (normpdf(Z(nn),0, 1))/abs(Dfw(nn));
end

toc
%str=input('Analytic Values calculated; Press a key to start monte carlo');

rng(29079137, 'twister')
paths=100000;
YY(1:paths)=x0;  %Original process monte carlo.
Random1(1:paths)=0;
YYMean(1:TtM)=0;
for tt=1:TtM
Random1=randn(size(Random1));
HermiteP1(1,1:paths)=1;
HermiteP1(2,1:paths)=Random1(1:paths);
HermiteP1(3,1:paths)=Random1(1:paths).^2-1;
HermiteP1(4,1:paths)=Random1(1:paths).^3-3*Random1(1:paths);
HermiteP1(5,1:paths)=Random1(1:paths).^4-6*Random1(1:paths).^2+3;

YY(1:paths)=YY(1:paths) + ...
(YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+ ...
YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
(YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+ ...
YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+ ...
YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+ ...
YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
(YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+ ...
YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5) .*HermiteP1(2,1:paths) + ...
((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
(YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+ ...
YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2).*HermiteP1(3,1:paths) + ...
((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )).*HermiteP1(4,1:paths) + ...
(YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);

%Uncomment for fourth order monte carlo

%
%   YY(1:paths)=YY(1:paths) + ...
%       (YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
%       YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
%       (YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
%       YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
%       YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
%       (YCoeff0(1,1,4,1).*YY(1:paths).^Fp(1,1,4,1)+YCoeff0(1,2,3,1).*YY(1:paths).^Fp(1,2,3,1)+ ...
%       YCoeff0(2,1,3,1).*YY(1:paths).^Fp(2,1,3,1)+YCoeff0(1,3,2,1).*YY(1:paths).^Fp(1,3,2,1)+ ...
%       YCoeff0(2,2,2,1).*YY(1:paths).^Fp(2,2,2,1)+YCoeff0(3,1,2,1).*YY(1:paths).^Fp(3,1,2,1)+ ...
%       YCoeff0(1,4,1,1).*YY(1:paths).^Fp(1,4,1,1)+YCoeff0(2,3,1,1).*YY(1:paths).^Fp(2,3,1,1)+ ...
%       YCoeff0(3,2,1,1).*YY(1:paths).^Fp(3,2,1,1)+YCoeff0(4,1,1,1).*YY(1:paths).^Fp(4,1,1,1))*dtM^3 + ...
%        (YCoeff0(1,1,5,1).*YY(1:paths).^Fp(1,1,5,1)+YCoeff0(1,2,4,1).*yy(1:paths).^Fp(1,2,4,1)+ ...
%     YCoeff0(2,1,4,1).*YY(1:paths).^Fp(2,1,4,1)+YCoeff0(2,1,1,1).*yy(1:paths).^Fp(2,1,1,1)+ ...
%      YCoeff0(2,2,3,1).*YY(1:paths).^Fp(2,2,3,1)+ ...
%      YCoeff0(3,1,3,1).*YY(1:paths).^Fp(3,1,3,1)+YCoeff0(1,4,2,1).*YY(1:paths).^Fp(1,4,2,1)+ ...
%      YCoeff0(2,3,2,1).*YY(1:paths).^Fp(2,3,2,1)+YCoeff0(3,2,2,1).*YY(1:paths).^Fp(3,2,2,1)+ ...
%      YCoeff0(4,1,2,1).*YY(1:paths).^Fp(4,1,2,1)+YCoeff0(1,5,1,1).*YY(1:paths).^Fp(1,5,1,1)+ ...
%      YCoeff0(2,4,1,1).*YY(1:paths).^Fp(2,4,1,1)+ ...
%      YCoeff0(3,3,1,1).*YY(1:paths).^Fp(3,3,1,1)+YCoeff0(4,2,1,1).*YY(1:paths).^Fp(4,2,1,1)+ ...
%      YCoeff0(5,1,1,1).*YY(1:paths).^Fp(5,1,1,1))*dtM^4+ ...
%       ((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
%     (YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
%     YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5+ ...
%     (YCoeff0(1,1,3,2).*YY(1:paths).^Fp(1,1,3,2)+YCoeff0(1,2,2,2).*YY(1:paths).^Fp(1,2,2,2)+ ...
%     YCoeff0(2,1,2,2).*YY(1:paths).^Fp(2,1,2,2)+YCoeff0(1,3,1,2).*YY(1:paths).^Fp(1,3,1,2)+ ...
%     YCoeff0(2,2,1,2).*YY(1:paths).^Fp(2,2,1,2)+YCoeff0(3,1,1,2).*YY(1:paths).^Fp(3,1,1,2)).*dtM^2.5+ ...
%     (YCoeff0(1,1,4,2).*YY(1:paths).^Fp(1,1,4,2)+YCoeff0(1,2,3,2).*YY(1:paths).^Fp(1,2,3,2)+ ...
%     YCoeff0(2,1,3,2).*YY(1:paths).^Fp(2,1,3,2)+YCoeff0(1,3,2,2).*YY(1:paths).^Fp(1,3,2,2)+ ...
%     YCoeff0(2,2,2,2).*YY(1:paths).^Fp(2,2,2,2)+ YCoeff0(3,1,2,2).*YY(1:paths).^Fp(3,1,2,2)+ ...
%     YCoeff0(1,4,1,2).*YY(1:paths).^Fp(1,4,1,2)+YCoeff0(2,3,1,2).*YY(1:paths).^Fp(2,3,1,2)+ ...
%     YCoeff0(3,2,1,2).*YY(1:paths).^Fp(3,2,1,2)+YCoeff0(4,1,1,2).*YY(1:paths).^Fp(4,1,1,2)).*dtM^3.5) .*HermiteP1(2,1:paths) + ...
%     ((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
%     (YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
%     YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2+ ...
%     (YCoeff0(1,1,3,3).*YY(1:paths).^Fp(1,1,3,3)+YCoeff0(1,2,2,3).*YY(1:paths).^Fp(1,2,2,3)+ ...
%     YCoeff0(2,1,2,3).*YY(1:paths).^Fp(2,1,2,3) + YCoeff0(1,3,1,3).*YY(1:paths).^Fp(1,3,1,3)+ ...
%     YCoeff0(2,2,1,3).*YY(1:paths).^Fp(2,2,1,3)+YCoeff0(3,1,1,3).*YY(1:paths).^Fp(3,1,1,3)).*dtM^3).*HermiteP1(3,1:paths) + ...
%     ((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )+ ...
%     (YCoeff0(1,1,2,4).*YY(1:paths).^Fp(1,1,2,4)+YCoeff0(1,2,1,4).*YY(1:paths).^Fp(1,2,1,4)+ ...
%     YCoeff0(2,1,1,4).*YY(1:paths).^Fp(2,1,1,4))*dtM^2.5).*HermiteP1(4,1:paths) + ...
%     (YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);
%

YYMean(tt)=YYMean(tt)+sum(YY(1:paths))/paths;
end
YY(YY<0)=0;
disp('Original process average from monte carlo');
MCMean=sum(YY(:))/paths %origianl coordinates monte carlo average.
disp('Original process average from our simulation');
ItoHermiteMean=sum(y_w(wnStart+1:Nn-1).*ZProb(wnStart+1:Nn-1)) %Original process average from coordinates

disp('true Mean only applicble to standard SV mean reverting type models otherwise disregard');
TrueMean=theta+(x0-theta)*exp(-kappa*dt*Tt)%Mean reverting SDE original variable true average

MaxCutOff=30;
NoOfBins=round(500*gamma^2*4*sigma0);
[YDensity,IndexOutY,IndexMaxY] = MakeDensityFromSimulation_Infiniti_NEW(YY,paths,NoOfBins,MaxCutOff );
plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g');
%plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g',Z(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'b');
title('Graphical Comparison of Ito-Hermite Method and Monte Carlo Method Density')
legend({'Ito-Hermite Density','Monte Carlo Density'},'Location','northeast')

str=input('red line is density of SDE from Ito-Hermite method, green is monte carlo.');

end


Here is another program you will need to run this program.
function [w0,Sigma00,Sigma11,Sigma22,Sigma33,Sigma1,Sigma2,Sigma3,Sigma0] = CalculateHermiteRepresentationOfDensityOrderThreeNew(w,Z,wnStart,Nn,dNn,NnMidh,NnMidl)

%This program find hermite representation of the SDE variable w to
%tjird Order as a function of three hermite polynomials.
%This program is not perfectly precise and has very small errors so you
%will have to be careful using it everywhere.
%Here wnStart is the starting grid point of the SDE vector
%mostly wnStart=1.
%Please note we are representing SDE Variable w as a function of hermite
%polynomials. It is not the density which can however be found by change of variable
%derivative of w with respect to Z using standard gaussian variable.
%Nn is the index of last density grid point.
%So w(wnStart:Nn) represents the SDE density variable grid.
%  Z(wnStart:Nn) represents the associated value of standard Gaussian on
%  the grid. w and Z can be associated by matching appropriate CDF points
%  on the grid.
%I suppose the grid is uniform in Z and dNn is the spacing between and
% Z(n) and Z(n+1) two points on the grid.

Z0=0;
w0= InterpolateOrderN6(6,Z0,Z(NnMidh+2),Z(NnMidh+1),Z(NnMidh),Z(NnMidl),Z(NnMidl-1),Z(NnMidl-2),w(NnMidh+2),w(NnMidh+1),w(NnMidh),w(NnMidl),w(NnMidl-1),w(NnMidl-2));

%Below Calculate the vol associated with each point on the grid
for nn=wnStart:Nn
Sigma0(nn)=(w(nn)-w0)./(Z(nn)-Z0);
end

%Below calculate the associated Sigma00 which is a single volatility associated with
%the first orthogonal hermite polynomial.

Sigma00= InterpolateOrderN(4,Z0,Z(NnMidh+1),Z(NnMidh),Z(NnMidl),Z(NnMidl-1),0,Sigma0(NnMidh+1),Sigma0(NnMidh),Sigma0(NnMidl),Sigma0(NnMidl-1),0);

%Below calculate the associated first derivative of Sigma0 with respect to
%Z on the grid.
dSigma0dZ(wnStart)=(-3*Sigma0(wnStart)+4*Sigma0(wnStart+1)-1*Sigma0(wnStart+2))/(2*dNn);
dSigma0dZ(wnStart+1:Nn-1)=(-1*Sigma0(wnStart:Nn-2)+1*Sigma0(wnStart+2:Nn))/(2*dNn);
dSigma0dZ(Nn)=(3*Sigma0(Nn)-4*Sigma0(Nn-1)+1*Sigma0(Nn-2))/(2*dNn);
dSigma0dZ(wnStart+2:Nn-2)= (1*Sigma0(wnStart:Nn-4)-8*Sigma0(wnStart+1:Nn-3)+0*Sigma0(wnStart+2:Nn-2)+8*Sigma0(wnStart+3:Nn-1)-1*Sigma0(wnStart+4:Nn))/(12*dNn);
dSigma0dZ0 = (-9*Sigma0(NnMidl-2)+125*Sigma0(NnMidl-1)-2250*Sigma0(NnMidl)+2250*Sigma0(NnMidh)-125*Sigma0(NnMidh+1)+9*Sigma0(NnMidh+2))/(3840*(.5*dNn));

%Following is the operation related to equation(4) in post 860.
SSigma1(NnMidh)=(.5*dSigma0dZ0+.5*dSigma0dZ(NnMidh))*dNn/2;
for nn=NnMidh+1:Nn
SSigma1(nn)=SSigma1(nn-1)+(.5*dSigma0dZ(nn-1)+.5*dSigma0dZ(nn))*dNn;
end

SSigma1(NnMidl)=-(.5*dSigma0dZ0+.5*dSigma0dZ(NnMidl))*dNn/2;
for nn=NnMidl-1:-1:wnStart
SSigma1(nn)=SSigma1(nn+1)-(.5*dSigma0dZ(nn+1)+.5*dSigma0dZ(nn))*dNn;
end

Sigma1(wnStart:Nn)=SSigma1(wnStart:Nn)./Z(wnStart:Nn);

Sigma11= InterpolateOrderN6(6,Z0,Z(NnMidh+2),Z(NnMidh+1),Z(NnMidh),Z(NnMidl),Z(NnMidl-1),Z(NnMidl-2),Sigma1(NnMidh+2),Sigma1(NnMidh+1),Sigma1(NnMidh),Sigma1(NnMidl),Sigma1(NnMidl-1),Sigma1(NnMidl-2));

%Now below we represent the SDE variable w in terms of first two hermite[/font][/size]
%polynomials and this associated value of w produced by hermite polynomials
%coefficients is called w1.

% w1(wnStart:Nn)=wMean-Sigma00*ZMean-Sigma1(wnStart:Nn).*(ZMean.^2-1)+ ...
% Sigma00*Z(wnStart:Nn)+ ...
% Sigma1(wnStart:Nn).*(Z(wnStart:Nn).^2-1);

w0(wnStart:Nn)=w0+Sigma0(wnStart:Nn).*Z(wnStart:Nn);

w1=w0;
w2=w0;
%w1(wnStart:Nn)=w0+ Sigma00*Z(wnStart:Nn)+ ...
% Sigma1(wnStart:Nn).*(Z(wnStart:Nn).^2-1);

%w2(wnStart:Nn)=w0+Sigma00*Z(wnStart:Nn)+ ...
% Sigma1(wnStart:Nn).*(Z(wnStart:Nn).^2-1);

dSigma1dZ(wnStart)=(-3*Sigma1(wnStart)+4*Sigma1(wnStart+1)-1*Sigma1(wnStart+2))/(2*dNn);
dSigma1dZ(wnStart+1:Nn-1)=(-1*Sigma1(wnStart:Nn-2)+1*Sigma1(wnStart+2:Nn))/(2*dNn);
dSigma1dZ(Nn)=(3*Sigma1(Nn)-4*Sigma1(Nn-1)+1*Sigma1(Nn-2))/(2*dNn);

dSigma1dZ0 = (-9*Sigma1(NnMidl-2)+125*Sigma1(NnMidl-1)-2250*Sigma1(NnMidl)+2250*Sigma1(NnMidh)-125*Sigma1(NnMidh+1)+9*Sigma1(NnMidh+2))/(3840*(.5*dNn));

SSigma2(NnMidh)=(.5*dSigma1dZ0+.5*dSigma1dZ(NnMidh))*dNn/2;
for nn=NnMidh+1:Nn
SSigma2(nn)=SSigma2(nn-1)+(.5*dSigma1dZ(nn-1)+.5*dSigma1dZ(nn))*dNn;
end
SSigma2(NnMidl)=-(.5*dSigma1dZ0+.5*dSigma1dZ(NnMidl))*dNn/2;
for nn=NnMidl-1:-1:wnStart
SSigma2(nn)=SSigma2(nn+1)-(.5*dSigma1dZ(nn+1)+.5*dSigma1dZ(nn))*dNn;
end

Sigma2(wnStart:Nn)=SSigma2(wnStart:Nn)./Z(wnStart:Nn);

Sigma22= InterpolateOrderN6(6,Z0,Z(NnMidh+2),Z(NnMidh+1),Z(NnMidh),Z(NnMidl),Z(NnMidl-1),Z(NnMidl-2),Sigma2(NnMidh+2),Sigma2(NnMidh+1),Sigma2(NnMidh),Sigma2(NnMidl),Sigma2(NnMidl-1),Sigma2(NnMidl-2));

dSigma2dZ(wnStart)=(-3*Sigma2(wnStart)+4*Sigma2(wnStart+1)-1*Sigma2(wnStart+2))/(2*dNn);
dSigma2dZ(wnStart+1:Nn-1)=(-1*Sigma2(wnStart:Nn-2)+1*Sigma2(wnStart+2:Nn))/(2*dNn);
dSigma2dZ(Nn)=(3*Sigma2(Nn)-4*Sigma2(Nn-1)+1*Sigma2(Nn-2))/(2*dNn);

dSigma2dZ0 = (-9*Sigma2(NnMidl-2)+125*Sigma2(NnMidl-1)-2250*Sigma2(NnMidl)+2250*Sigma2(NnMidh)-125*Sigma2(NnMidh+1)+9*Sigma2(NnMidh+2))/(3840*(.5*dNn));

SSigma3(NnMidh)=(.5*dSigma2dZ0+.5*dSigma2dZ(NnMidh))*dNn/2;
for nn=NnMidh+1:Nn
SSigma3(nn)=SSigma3(nn-1)+(.5*dSigma2dZ(nn-1)+.5*dSigma2dZ(nn))*dNn;
end
SSigma3(NnMidl)=-(.5*dSigma2dZ0+.5*dSigma2dZ(NnMidl))*dNn/2;
for nn=NnMidl-1:-1:wnStart
SSigma3(nn)=SSigma3(nn+1)-(.5*dSigma2dZ(nn+1)+.5*dSigma2dZ(nn))*dNn;
end

Sigma3(wnStart:Nn)=SSigma3(wnStart:Nn)./Z(wnStart:Nn);

Sigma33= InterpolateOrderN6(6,Z0,Z(NnMidh+2),Z(NnMidh+1),Z(NnMidh),Z(NnMidl),Z(NnMidl-1),Z(NnMidl-2),Sigma3(NnMidh+2),Sigma3(NnMidh+1),Sigma3(NnMidh),Sigma3(NnMidl),Sigma3(NnMidl-1),Sigma3(NnMidl-2));

end
And you will need this drift and volatility calculation program
function [wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,c1,c2,c3,c4] = CalculateDriftAndVolOrder4(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt)

yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

Fp2=Fp1/(1-gamma);

wMu0dt(wnStart:Nn)=(YqCoeff0(1,1,2,1).*yy(wnStart:Nn).^Fp1(1,1,2,1)+ ...
YqCoeff0(1,2,1,1).*yy(wnStart:Nn).^Fp1(1,2,1,1)+ ...
YqCoeff0(2,1,1,1).*yy(wnStart:Nn).^Fp1(2,1,1,1))*dt + ...
(YqCoeff0(1,1,3,1).*yy(wnStart:Nn).^Fp1(1,1,3,1)+ ...
YqCoeff0(1,2,2,1).*yy(wnStart:Nn).^Fp1(1,2,2,1)+ ...
YqCoeff0(2,1,2,1).*yy(wnStart:Nn).^Fp1(2,1,2,1)+ ...
YqCoeff0(1,3,1,1).*yy(wnStart:Nn).^Fp1(1,3,1,1)+ ...
YqCoeff0(2,2,1,1).*yy(wnStart:Nn).^Fp1(2,2,1,1)+ ...
YqCoeff0(3,1,1,1).*yy(wnStart:Nn).^Fp1(3,1,1,1))*dt^2 + ...
(YqCoeff0(1,1,4,1).*yy(wnStart:Nn).^Fp1(1,1,4,1)+ ...
YqCoeff0(1,2,3,1).*yy(wnStart:Nn).^Fp1(1,2,3,1)+ ...
YqCoeff0(2,1,3,1).*yy(wnStart:Nn).^Fp1(2,1,3,1)+ ...
YqCoeff0(1,3,2,1).*yy(wnStart:Nn).^Fp1(1,3,2,1)+ ...
YqCoeff0(2,2,2,1).*yy(wnStart:Nn).^Fp1(2,2,2,1)+ ...
YqCoeff0(3,1,2,1).*yy(wnStart:Nn).^Fp1(3,1,2,1)+ ...
YqCoeff0(1,4,1,1).*yy(wnStart:Nn).^Fp1(1,4,1,1)+ ...
YqCoeff0(2,3,1,1).*yy(wnStart:Nn).^Fp1(2,3,1,1)+ ...
YqCoeff0(3,2,1,1).*yy(wnStart:Nn).^Fp1(3,2,1,1)+ ...
YqCoeff0(4,1,1,1).*yy(wnStart:Nn).^Fp1(4,1,1,1))*dt^3;

dwMu0dtdw(wnStart:Nn)=(YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,1,1)))*dt + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,1,1,1)))*dt^2 + ...
(YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,4,1))+ ...
YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,3,1))+ ...
YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,3,1))+ ...
YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,3,2,1))+ ...
YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,2,2,1))+ ...
YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,1,2,1))+ ...
YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,4,1,1))+ ...
YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,3,1,1))+ ...
YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,2,1,1))+ ...
YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(4,1,1,1)))*dt^3;

d2wMu0dtdw2(wnStart:Nn)=(1-gamma).*((YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*(-1+Fp2(1,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*(-1+Fp2(1,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*(-1+Fp2(2,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,1,1)))*dt + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*(-1+Fp2(1,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*(-1+Fp2(1,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*(-1+Fp2(2,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*(-1+Fp2(1,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*(-1+Fp2(2,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*(-1+Fp2(3,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,1,1,1)))*dt^2 + ...
(YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*(-1+Fp2(1,1,4,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,4,1))+ ...
YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*(-1+Fp2(1,2,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,3,1))+ ...
YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*(-1+Fp2(2,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,3,1))+ ...
YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*(-1+Fp2(1,3,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,3,2,1))+ ...
YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*(-1+Fp2(2,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,2,2,1))+ ...
YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*(-1+Fp2(3,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,1,2,1))+ ...
YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*(-1+Fp2(1,4,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,4,1,1))+ ...
YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*(-1+Fp2(2,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,3,1,1))+ ...
YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*(-1+Fp2(3,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,2,1,1))+ ...
YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*(-1+Fp2(4,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(4,1,1,1)))*dt^3);

d3wMu0dtdw3(wnStart:Nn)=(1-gamma)^2.*((YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*(-1+Fp2(1,1,2,1)).*(-2+Fp2(1,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*(-1+Fp2(1,2,1,1)).*(-2+Fp2(1,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*(-1+Fp2(2,1,1,1)).*(-2+Fp2(2,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,1,1)))*dt + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*(-1+Fp2(1,1,3,1)).*(-2+Fp2(1,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*(-1+Fp2(1,2,2,1)).*(-2+Fp2(1,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*(-1+Fp2(2,1,2,1)).*(-2+Fp2(2,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*(-1+Fp2(1,3,1,1)).*(-2+Fp2(1,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*(-1+Fp2(2,2,1,1)).*(-2+Fp2(2,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*(-1+Fp2(3,1,1,1)).*(-2+Fp2(3,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,1,1,1)))*dt^2 + ...
(YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*(-1+Fp2(1,1,4,1)).*(-2+Fp2(1,1,4,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,4,1))+ ...
YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*(-1+Fp2(1,2,3,1)).*(-2+Fp2(1,2,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,3,1))+ ...
YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*(-1+Fp2(2,1,3,1)).*(-2+Fp2(2,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,3,1))+ ...
YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*(-1+Fp2(1,3,2,1)).*(-2+Fp2(1,3,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,3,2,1))+ ...
YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*(-1+Fp2(2,2,2,1)).*(-2+Fp2(2,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,2,2,1))+ ...
YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*(-1+Fp2(3,1,2,1)).*(-2+Fp2(3,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,1,2,1))+ ...
YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*(-1+Fp2(1,4,1,1)).*(-2+Fp2(1,4,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,4,1,1))+ ...
YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*(-1+Fp2(2,3,1,1)).*(-2+Fp2(2,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,3,1,1))+ ...
YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*(-1+Fp2(3,2,1,1)).*(-2+Fp2(3,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,2,1,1))+ ...
YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*(-1+Fp2(4,1,1,1)).*(-2+Fp2(4,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(4,1,1,1)))*dt^3);

c1(wnStart:Nn)=YqCoeff0(1,1,1,2).*yy(wnStart:Nn).^Fp1(1,1,1,2).*sqrt(dt)+ ...
(YqCoeff0(1,1,2,2).*yy(wnStart:Nn).^Fp1(1,1,2,2)+YqCoeff0(1,2,1,2).*yy(wnStart:Nn).^Fp1(1,2,1,2)+ ...
YqCoeff0(2,1,1,2).*yy(wnStart:Nn).^Fp1(2,1,1,2)).*dt^1.5+ ...
(YqCoeff0(1,1,3,2).*yy(wnStart:Nn).^Fp1(1,1,3,2)+YqCoeff0(1,2,2,2).*yy(wnStart:Nn).^Fp1(1,2,2,2)+ ...
YqCoeff0(2,1,2,2).*yy(wnStart:Nn).^Fp1(2,1,2,2)+YqCoeff0(1,3,1,2).*yy(wnStart:Nn).^Fp1(1,3,1,2)+ ...
YqCoeff0(2,2,1,2).*yy(wnStart:Nn).^Fp1(2,2,1,2)+YqCoeff0(3,1,1,2).*yy(wnStart:Nn).^Fp1(3,1,1,2)).*dt^2.5;

c2(wnStart:Nn)=YqCoeff0(1,1,1,3).*yy(wnStart:Nn).^Fp1(1,1,1,3) *dt + ...
(YqCoeff0(1,1,2,3).*yy(wnStart:Nn).^Fp1(1,1,2,3)+YqCoeff0(1,2,1,3).*yy(wnStart:Nn).^Fp1(1,2,1,3)+ ...
YqCoeff0(2,1,1,3).*yy(wnStart:Nn).^Fp1(2,1,1,3)).*dt^2+ ...
(YqCoeff0(1,1,3,3).*yy(wnStart:Nn).^Fp1(1,1,3,3)+YqCoeff0(1,2,2,3).*yy(wnStart:Nn).^Fp1(1,2,2,3)+ ...
YqCoeff0(2,1,2,3).*yy(wnStart:Nn).^Fp1(2,1,2,3) + YqCoeff0(1,3,1,3).*yy(wnStart:Nn).^Fp1(1,3,1,3)+ ...
YqCoeff0(2,2,1,3).*yy(wnStart:Nn).^Fp1(2,2,1,3)+YqCoeff0(3,1,1,3).*yy(wnStart:Nn).^Fp1(3,1,1,3)).*dt^3;

c3(wnStart:Nn)=YqCoeff0(1,1,1,4).*yy(wnStart:Nn).^Fp1(1,1,1,4)*dt^1.5 + ...
(YqCoeff0(1,1,2,4).*yy(wnStart:Nn).^Fp1(1,1,2,4)+YqCoeff0(1,2,1,4).*yy(wnStart:Nn).^Fp1(1,2,1,4)+ ...
YqCoeff0(2,1,1,4).*yy(wnStart:Nn).^Fp1(2,1,1,4))*dt^2.5;

c4(wnStart:Nn)=YqCoeff0(1,1,1,5).*yy(wnStart:Nn).^Fp1(1,1,1,5)*dt^2.0;

end

when you run this program on command line with following parameters
[TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected04(.1,.4,1.0,.95,1,4)
you will get Ito-hermite mean=0.394460900874346
and MC Mean=0.392725376643723

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

Friends, the last Ito-hermite density simulation program I posted had minor problems as density was getting clipped at the start for smaller volatility exponents when sigma was high. I have improved it to a great extent and now it works much better. Though gamma=.5 still remains problematic close to zero, we get very good results with gamma>.65 even close to zero with reasonable volatility. It is a much improved program as the problem of density being clipped at the start has become very insignificant now.
To improve the program, I have multiplied the correction terms with scaled normal exponential and it improves the analytic density on both extremes as the density being clipped becomes very little and the problem of huge volatility on positive normal extreme also greatly decreases. Now you can probably also try numerical derivatives which were being unstable earlier since they were not multiplied with normal exponential. Though I have yet not tried numerical derivatives(FD derivatives), I invite friends to experiment with them.
Though when mean is known, the results are very good even now, I am very sure, once we tackle the mean reverting type SDEs with time dependent exponential multiplication method that reduces one term in the drift, we will get very excellent results.
Here is the new version of the program.
function [TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected04(x0,theta,kappa,gamma,sigma0,T)

%Copyright Ahsan Amin. Infiniti derivatives Technologies.
%or skype ahsan.amin2999
%In this program, I am simulating the SDE given as
%dy(t)=mu1 x(t)^beta1 dt + mu2 x(t)^beta2 dt +sigma x(t)^gamma dz(t)

%I have not directly simulated the SDE but simulated the transformed
%Besse1l process version of the SDE and then changed coordinates to retreive
%the SDE in original coo
%rdinates.
%The present program will analytically evolve only the Bessel Process version of the
%SDE in transformed coordinates.
%In this version, the mean correction has been disabled. If you are
%simulating standard mean-reverting SV type SDEs, please enable the mean
%correction by uncommenting the appropriate line in the body of code below.

dt=.125/16;   % Simulation time interval.%Fodiffusions close to zero
%decrease dt for accuracy.
%T=5;
Tt=128*T;%16*16;     % Number of simulation levels. Terminal time= Tt*dt; //.125/32*32*16=2 year;
OrderA=4;  %
OrderM=4;  %
%dtM=.125/8;%Monte carlo time interval size dtM.
%TtM=8*8;%Monte carlo number of simulation intervals.
dtM=dt*2;
TtM=Tt/2;

dNn=.2/1;   % Normal density subdivisions width. would change with number of subdivisions
Nn=48;  % No of normal density subdivisions
NnMidl=25;%One half density Subdivision left from mid of normal density(low)
NnMidh=26;%One half density subdivision right from the mid of normal density(high)
NnMid=4.0;

%theta=mu1/(-mu2);
%kappa=-mu2;

%x0=.25;   % starting value of SDE
beta1=0.0;
beta2=1.0;   % Second drift term power.
%gamma=.95;%50;   % volatility power.
%kappa=.5;   %mean reversion parameter.
%theta=.075;%mean reversion target

%you can specify any general mu1 and mu2 and beta1 and beta2.
%The present program is not perfect but quite reasonably good.
%if there is only one term in drift with beta=0, there will be minor
%errors. This case is yet not covered.

mu1=+1*theta*kappa;   %first drift coefficient.
mu2=-1*kappa;    % Second drift coefficient.
%sigma0=1.0;%Volatility value
alpha=1;% x^alpha is being expanded. This is currently for monte carlo only.
alpha1=1-gamma;

w(1:Nn)=x0^(1-gamma)/(1-gamma);

Z(1:Nn)=(((1:Nn)-5.5)*dNn-NnMid);

Z
str=input('Look at Z');
ZProb(1)=normcdf(.5*Z(1)+.5*Z(2),0,1)-normcdf(.5*Z(1)+.5*Z(2)-dNn,0,1);
ZProb(Nn)=normcdf(.5*Z(Nn)+.5*Z(Nn-1)+dNn,0,1)-normcdf(.5*Z(Nn)+.5*Z(Nn-1),0,1);

for nn=2:Nn-1
ZProb(nn)=normcdf(.5*Z(nn)+.5*Z(nn+1),0,1)-normcdf(.5*Z(nn)+.5*Z(nn-1),0,1);
%Above calculate probability mass in each probability subdivision.
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

sigma11(1:OrderA+1)=0;
mu11(1:OrderA+1)=0;
mu22(1:OrderA+1)=0;
sigma22(1:OrderA+1)=0;
% index 1 correponds to zero level since matlab indexing starts at one.
sigma11(1)=1;
mu11(1)=1;
mu22(1)=1;
sigma22(1)=1;

for k=1:(OrderA+1)
if sigma0~=0
sigma11(k)=sigma0^(k-1);
end
if mu1 ~= 0
mu11(k)=mu1^(k-1);
end
if mu2 ~= 0
mu22(k)=mu2^(k-1);
end
if sigma0~=0
sigma22(k)=sigma0^(2*(k-1));
end
end
%Ft(1:TtM+1,1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0; %General time powers on hermite polynomials
Fp(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers on coefficients of hermite polynomials.
Fp1(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers for bessel transformed coordinates.

%YCoeff0 and YCoeff are coefficents for original coordinates monte carlo.
%YqCoeff0 and YqCoeff are bessel/lamperti version monte carlo.

YCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
YqCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
%Pre-compute the time and power exponent values in small multi-dimensional arrays
YCoeff = ItoTaylorCoeffsNew(alpha,beta1,beta2,gamma); %expand y^alpha where alpha=1;
YqCoeff = ItoTaylorCoeffsNew(alpha1,beta1,beta2,gamma);%expand y^alpha1 where alpha1=(1-gamma)
YqCoeff=YqCoeff/(1-gamma); %Transformed coordinates coefficients have to be
%further divided by (1-gamma)

for k = 0 : (OrderA)
for m = 0:k
l4 = k - m + 1;
for n = 0 : m
l3 = m - n + 1;
for j = 0:n
l2 = n - j + 1;
l1 = j + 1;
%Ft(l1,l2,l3,l4) = dtM^((l1-1) + (l2-1) + (l3-1) + .5* (l4-1));
Fp(l1,l2,l3,l4) = (alpha + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));
Fp1(l1,l2,l3,l4) = (alpha1 + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));

YCoeff0(l1,l2,l3,l4) =YCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
YqCoeff0(l1,l2,l3,l4) =YqCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
end
end
end
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
wnStart=1;%
d2wdZ2(1:Nn)=0;
dwdZ(1:Nn)=0;
yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
tic

for tt=1:Tt

yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

[wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,c1,c2,c3,c4] = CalculateDriftAndVolOrder4(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt);

dw(wnStart:Nn)=c1(wnStart:Nn).*Z(wnStart:Nn) ;% ...
dw2(wnStart:Nn)=dw(wnStart:Nn).^2;
% d2w(wnStart:Nn)=c2(wnStart:Nn).*(Z(wnStart:Nn).^2-1);
% d3w(wnStart:Nn)=c3(wnStart:Nn).*(Z(wnStart:Nn).^3-3*Z(wnStart:Nn));
w(isnan(w)==1)=0;
wMu0dt(isnan(wMu0dt)==1)=0;
wMeanPrev=sum(ZProb(wnStart:Nn).*w(wnStart:Nn));%Calculate the mean.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

[w0,Sigma00,Sigma11,Sigma22,Sigma33,Sigma1,Sigma2,Sigma3,Sigma0] = CalculateHermiteRepresentationOfDensityOrderThreeNew(w,Z,wnStart,Nn,dNn,NnMidh,NnMidl);
d2wdZ2(wnStart:Nn)=d2wdZ2(wnStart:Nn)+ d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2+dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn);%+ ...

ExpDamp(wnStart:Nn)=1.0;
ExpDamp(wnStart:NnMidl)=exp(-.025*Z(wnStart:NnMidl).^2);

dwdZ(wnStart:Nn)=(Sigma00)+Sigma1(wnStart:Nn).*2.*Z(wnStart:Nn);%.*ExpDamp(wnStart:Nn);

H2(wnStart:Nn)=(Z(wnStart:Nn).^2-1);
CTerm(wnStart:Nn)=(.5*d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2 - ...
1*dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn));
Correction1(wnStart:Nn)=-sqrt(2/3)/(22/7)^2.*CTerm(wnStart:Nn).*H2(wnStart:Nn);
Correction0(wnStart:Nn)=-sqrt(2/3)/(22/7)^2.*(d2wdZ2(wnStart:Nn)-.25*dwdZ(wnStart:Nn).^2).*H2(wnStart:Nn);

if(tt==1)
w(wnStart:Nn)=wMeanPrev+wMu0dt(wnStart:Nn)+ ...
sign(w(wnStart:Nn)-wMeanPrev+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn)));
end
if(tt>1)

w(wnStart:Nn)=wMeanPrev+wMu0dt(wnStart:Nn)+ ...
sign(w(wnStart:Nn)-wMeanPrev+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn)))+ ...
+dt *Correction0(wnStart:Nn).*exp(-.5*(1-gamma)*sigma0.^2.*Z(wnStart:Nn).^2) + ...
Correction1(wnStart:Nn).*exp(-.5*(1-gamma)*sigma0.^2.*Z(wnStart:Nn).^2);

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Below are instability correction equations. There is instability in the
% SDE representation equation roughly between -.5 SD to +.5 SD in normal terms.
%Instability correction Algorithm takes two ends that are here given as
%lower end=w(NnMidl-2) and upper end=w(NnMidh+3).
%In the block below the interpolation starts from lower end.
%1st equation is
%And so on.
%There is a sixth equation that is not written below(since it takes us to
%the upper interpolation point which is already known) which is
%Sum of these six equations add up to the distance between upper and lower
%interpolation points.
%In the sum there are six instances of first derivative which is already
%calculated at the boundary, six instances of second derivative which is also
%already calculated at lower boundary and 18.0 instances of DeltaD1w which is
%unknown. This equation calculates the value of one unit of DeltaD1w addition
% I hope to find a better analytical method to tackle this issue. Friends
% are welcome to play with slightly changed related alternatives to the
%above interpolation.
CTheta=1.0;
D1wl=(11/6*w(NnMidl-2)-3*w(NnMidl-3)+3/2*w(NnMidl-4)-1/3*w(NnMidl-5))/dNn;
D2wl=(35/12*w(NnMidl-2) -26/3*w(NnMidl-3)+ 19/2*w(NnMidl-4)-14/3*w(NnMidl-5)+11/12*w(NnMidl-6))/dNn.^2;

%The last equation is not used since point w(NnMidh+3)is given but it is used in
%calculation of distance between two interpolation points. This distance is
%used to determine the value of DeltaD1w.

D1wh=(-11/6*w(NnMidh+3)+3*w(NnMidh+4)-3/2*w(NnMidh+5)+1/3*w(NnMidh+6))/dNn;
D2wh=(35/12*w(NnMidh+3) -26/3*w(NnMidh+4)+ 19/2*w(NnMidh+5)-14/3*w(NnMidh+6)+11/12*w(NnMidh+7))/dNn.^2;

%
w(NnMidh+2)=.5*whh(NnMidh+2)+.5*wll(NnMidh+2);
w(NnMidh+1)=.5*whh(NnMidh+1)+.5*wll(NnMidh+1);
w(NnMidh)=.5*whh(NnMidh)+.5*wll(NnMidh);
w(NnMidl)=.5*whh(NnMidl)+.5*wll(NnMidl);
w(NnMidl-1)=.5*whh(NnMidl-1)+.5*wll(NnMidl-1);

w1(1:Nn-1)=w(1:Nn-1);
w2(1:Nn-1)=w(2:Nn);

%         w1(1:Nn-2)=w(1:Nn-2);
%         w2(1:Nn-2)=w(2:Nn-1);
%         w3(1:Nn-2)=w(3:Nn);
%         w(w1(:)>w2(:))=w2(w1(:)>w2(:))-.25*(w3(w1(:)>w2(:))-w2(w1(:)>w2(:)));
w(w1(:)>w2(:))=0;%Be careful;might not universally hold;

w(w<0)=0.0;
for nn=1:Nn
if(w(nn)<=0)
wnStart=nn+1;
end
end

%%1st order mean correction. We know the mean in original
%%coordinates so I shift the density into original coordinates,
%%apply the mean correction and then transform again into Lamperti
%%coordinates. Algebra solves two equations in two unknowns.
%%Equations are Sum_0^N{(Y_w(wnStart:Nn)-Y0)*W0}=1 and the second
%%equation is Sum_0^N{Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0}=u0
%%Two unknows are Y0 and W0. u0 is known mean.
tt;
u0=theta+(x0-theta)*exp(-kappa*(tt*dt)); %analytic mean of the density

%If you are not using stochastic volatility, replace above with
%true mean otherwise results would become garbage.

Y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

Yn2Pn=sum(Y_w(wnStart:Nn).*Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
YnPn=sum(Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
Pn=1.0;%%Sum(ZProb(1:Nn))
Y0=(Yn2Pn-u0*YnPn)/(YnPn-u0*Pn);

Y0Pn=Y0*Pn;
W0=1/(YnPn-Y0Pn);
YCorrected(wnStart:Nn)=Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0;
wCorrected(wnStart:Nn)=(YCorrected(wnStart:Nn).^(1-gamma))./(1-gamma);
w(wnStart:Nn)=wCorrected(wnStart:Nn);

%I have disabled the mean correction. The above mean correction is only valid for
%standard mean reverting stochastic volatility type SDEs. To enable mean
%correction please uncomment the above last line in the block.

end

y_w(1:Nn)=0;
y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
Dfy_w(wnStart:Nn)=0;
Dfw(wnStart:Nn)=0;
for nn=wnStart+1:Nn-1
Dfy_w(nn) = (y_w(nn + 1) - y_w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
Dfw(nn) = (w(nn + 1) - w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
%Change of variable derivative for densities
end
fy_w(1:Nn)=0;
fw(1:Nn)=0;
for nn = wnStart:Nn-1
fy_w(nn) = (normpdf(Z(nn),0, 1))/abs(Dfy_w(nn));%Origianl coordinates density
fw(nn) = (normpdf(Z(nn),0, 1))/abs(Dfw(nn));
end

toc
%str=input('Analytic Values calculated; Press a key to start monte carlo');

rng(29079137, 'twister')
paths=100000;
YY(1:paths)=x0;  %Original process monte carlo.
Random1(1:paths)=0;
YYMean(1:TtM)=0;
for tt=1:TtM
Random1=randn(size(Random1));
HermiteP1(1,1:paths)=1;
HermiteP1(2,1:paths)=Random1(1:paths);
HermiteP1(3,1:paths)=Random1(1:paths).^2-1;
HermiteP1(4,1:paths)=Random1(1:paths).^3-3*Random1(1:paths);
HermiteP1(5,1:paths)=Random1(1:paths).^4-6*Random1(1:paths).^2+3;

YY(1:paths)=YY(1:paths) + ...
(YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+ ...
YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
(YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+ ...
YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+ ...
YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+ ...
YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
(YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+ ...
YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5) .*HermiteP1(2,1:paths) + ...
((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
(YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+ ...
YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2).*HermiteP1(3,1:paths) + ...
((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )).*HermiteP1(4,1:paths) + ...
(YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);

%Uncomment for fourth order monte carlo

%
%   YY(1:paths)=YY(1:paths) + ...
%       (YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
%       YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
%       (YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
%       YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
%       YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
%       (YCoeff0(1,1,4,1).*YY(1:paths).^Fp(1,1,4,1)+YCoeff0(1,2,3,1).*YY(1:paths).^Fp(1,2,3,1)+ ...
%       YCoeff0(2,1,3,1).*YY(1:paths).^Fp(2,1,3,1)+YCoeff0(1,3,2,1).*YY(1:paths).^Fp(1,3,2,1)+ ...
%       YCoeff0(2,2,2,1).*YY(1:paths).^Fp(2,2,2,1)+YCoeff0(3,1,2,1).*YY(1:paths).^Fp(3,1,2,1)+ ...
%       YCoeff0(1,4,1,1).*YY(1:paths).^Fp(1,4,1,1)+YCoeff0(2,3,1,1).*YY(1:paths).^Fp(2,3,1,1)+ ...
%       YCoeff0(3,2,1,1).*YY(1:paths).^Fp(3,2,1,1)+YCoeff0(4,1,1,1).*YY(1:paths).^Fp(4,1,1,1))*dtM^3 + ...
%        (YCoeff0(1,1,5,1).*YY(1:paths).^Fp(1,1,5,1)+YCoeff0(1,2,4,1).*yy(1:paths).^Fp(1,2,4,1)+ ...
%     YCoeff0(2,1,4,1).*YY(1:paths).^Fp(2,1,4,1)+YCoeff0(2,1,1,1).*yy(1:paths).^Fp(2,1,1,1)+ ...
%      YCoeff0(2,2,3,1).*YY(1:paths).^Fp(2,2,3,1)+ ...
%      YCoeff0(3,1,3,1).*YY(1:paths).^Fp(3,1,3,1)+YCoeff0(1,4,2,1).*YY(1:paths).^Fp(1,4,2,1)+ ...
%      YCoeff0(2,3,2,1).*YY(1:paths).^Fp(2,3,2,1)+YCoeff0(3,2,2,1).*YY(1:paths).^Fp(3,2,2,1)+ ...
%      YCoeff0(4,1,2,1).*YY(1:paths).^Fp(4,1,2,1)+YCoeff0(1,5,1,1).*YY(1:paths).^Fp(1,5,1,1)+ ...
%      YCoeff0(2,4,1,1).*YY(1:paths).^Fp(2,4,1,1)+ ...
%      YCoeff0(3,3,1,1).*YY(1:paths).^Fp(3,3,1,1)+YCoeff0(4,2,1,1).*YY(1:paths).^Fp(4,2,1,1)+ ...
%      YCoeff0(5,1,1,1).*YY(1:paths).^Fp(5,1,1,1))*dtM^4+ ...
%       ((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
%     (YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
%     YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5+ ...
%     (YCoeff0(1,1,3,2).*YY(1:paths).^Fp(1,1,3,2)+YCoeff0(1,2,2,2).*YY(1:paths).^Fp(1,2,2,2)+ ...
%     YCoeff0(2,1,2,2).*YY(1:paths).^Fp(2,1,2,2)+YCoeff0(1,3,1,2).*YY(1:paths).^Fp(1,3,1,2)+ ...
%     YCoeff0(2,2,1,2).*YY(1:paths).^Fp(2,2,1,2)+YCoeff0(3,1,1,2).*YY(1:paths).^Fp(3,1,1,2)).*dtM^2.5+ ...
%     (YCoeff0(1,1,4,2).*YY(1:paths).^Fp(1,1,4,2)+YCoeff0(1,2,3,2).*YY(1:paths).^Fp(1,2,3,2)+ ...
%     YCoeff0(2,1,3,2).*YY(1:paths).^Fp(2,1,3,2)+YCoeff0(1,3,2,2).*YY(1:paths).^Fp(1,3,2,2)+ ...
%     YCoeff0(2,2,2,2).*YY(1:paths).^Fp(2,2,2,2)+ YCoeff0(3,1,2,2).*YY(1:paths).^Fp(3,1,2,2)+ ...
%     YCoeff0(1,4,1,2).*YY(1:paths).^Fp(1,4,1,2)+YCoeff0(2,3,1,2).*YY(1:paths).^Fp(2,3,1,2)+ ...
%     YCoeff0(3,2,1,2).*YY(1:paths).^Fp(3,2,1,2)+YCoeff0(4,1,1,2).*YY(1:paths).^Fp(4,1,1,2)).*dtM^3.5) .*HermiteP1(2,1:paths) + ...
%     ((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
%     (YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
%     YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2+ ...
%     (YCoeff0(1,1,3,3).*YY(1:paths).^Fp(1,1,3,3)+YCoeff0(1,2,2,3).*YY(1:paths).^Fp(1,2,2,3)+ ...
%     YCoeff0(2,1,2,3).*YY(1:paths).^Fp(2,1,2,3) + YCoeff0(1,3,1,3).*YY(1:paths).^Fp(1,3,1,3)+ ...
%     YCoeff0(2,2,1,3).*YY(1:paths).^Fp(2,2,1,3)+YCoeff0(3,1,1,3).*YY(1:paths).^Fp(3,1,1,3)).*dtM^3).*HermiteP1(3,1:paths) + ...
%     ((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )+ ...
%     (YCoeff0(1,1,2,4).*YY(1:paths).^Fp(1,1,2,4)+YCoeff0(1,2,1,4).*YY(1:paths).^Fp(1,2,1,4)+ ...
%     YCoeff0(2,1,1,4).*YY(1:paths).^Fp(2,1,1,4))*dtM^2.5).*HermiteP1(4,1:paths) + ...
%     (YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);
%

YYMean(tt)=YYMean(tt)+sum(YY(1:paths))/paths;
end
YY(YY<0)=0;
disp('Original process average from monte carlo');
MCMean=sum(YY(:))/paths %origianl coordinates monte carlo average.
disp('Original process average from our simulation');
ItoHermiteMean=sum(y_w(wnStart+1:Nn-1).*ZProb(wnStart+1:Nn-1)) %Original process average from coordinates

disp('true Mean only applicble to standard SV mean reverting type models otherwise disregard');
TrueMean=theta+(x0-theta)*exp(-kappa*dt*Tt)%Mean reverting SDE original variable true average

MaxCutOff=30;
NoOfBins=round(500*gamma^2*4*sigma0);
[YDensity,IndexOutY,IndexMaxY] = MakeDensityFromSimulation_Infiniti_NEW(YY,paths,NoOfBins,MaxCutOff );
plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g');
%plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g',Z(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'b');
title('Graphical Comparison of Ito-Hermite Method and Monte Carlo Method Density')
legend({'Ito-Hermite Density','Monte Carlo Density'},'Location','northeast')

str=input('red line is density of SDE from Ito-Hermite method, green is monte carlo.');

end


Here is the check on the program. When you run it with the following parameters on command line
[TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected08(.2,.2,1,.75,.8,1)
you will get the following results:
ItoHermiteMean = 0.199990607186250
MCMean = 0.200351914578059

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

Here is the graph associated with the output of the parameters I posted in previous post. The parameters aer
x0=.2
theta=.2
kappa=1
gamma=.75
sigma=.8
T=1.0

Here is the graph

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

I am uploading graphical comparison of Ito-hermite density and monte carlo density for 12 different cases. The comparison has been done with the program in post 905. I have removed the portion of the graph where our density becomes indistinguishable from x-axis to focus on the main body of the density. You can very easily change the above program for any general SDE with two drift terms. Here is the  SDE
$dX(t)=\kappa (\theta - X(t)) dt + \sigma X(t)^{\gamma} dz(t), X(0)=x0$

I will be further improving the program in next few days and update here so you would be able to download the new program in a few days.

Please note that all the Ito-hermite densities shown in graphs below came from mean-corrected algorithm which ensures that mean of the Ito-hermite density is precisely the same as analytic mean of the density so tail that is not shown in many graphs is also extremely exact.
The parameters in the SDE are shown on the title of each graph.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

This is for friends who want to solve the mean reverting stochastic volatility type equations with Ito-hermite method of moving and expanding/contracting grid. When we have the equation given as

$dx(t)=\kappa (\theta - x(t)) dt + \sigma x^{\gamma} dz(t)$

We used a transformed version of the above equation and simulated the dynamics of $w(t)=\frac{x(t)^{1-\gamma}}{(1-\gamma)}$. In original coordinates the dynamics of evolution of w(t) are given as

$dw(t)=\kappa \theta x^{-\gamma} dt - \kappa x^{1-\gamma} dt -.5 \gamma {\sigma}^2 x^{(\gamma-1)} dt + \sigma dz(t)$

Just like for mean reverting equations in original coordinates, we can try a suitable choice of exponential and decrease the number of drift terms from three to only two. And we know that our Ito-hermite method works remarkably well when there are only two terms in the drift. So here is the new process we simulate

$d[\exp(\kappa(1-\gamma) t) w(t)]=d[\exp(\kappa(1-\gamma) t) \frac{x(t)^{1-\gamma}}{(1-\gamma)}]$
$=\exp(\kappa(1-\gamma) t) [\kappa \theta x^{-\gamma} dt - \kappa x^{1-\gamma} dt -.5 \gamma {\sigma}^2 x^{(\gamma-1)} dt + \sigma dz(t)]$
$+\exp(\kappa(1-\gamma) t) \kappa x^{1-\gamma} dt$
once we cancel the identical terms with opposite signs, we get the equation
$d[\exp(\kappa(1-\gamma) t) w(t)]$
$=\exp(\kappa(1-\gamma) t) [\kappa \theta x^{-\gamma} dt -.5 \gamma {\sigma}^2 x^{(\gamma-1)} dt + \sigma dz(t)]$
solving the above equation in proper integral form, we get
$w(t)=\exp(-\kappa(1-\gamma) t) w(0)$
$+\exp(-\kappa(1-\gamma) t) \int_0^t \exp(\kappa(1-\gamma) s) [\kappa \theta x^{-\gamma} -.5 \gamma {\sigma}^2 x^{(\gamma-1)}] ds$
$+\exp(-\kappa(1-\gamma) t) \int_0^t \exp(\kappa(1-\gamma) s) \sigma dz(s)$

So we have decreased the number of drift terms from three to two in exchange for time dependence in the SDE. If we could solve for time exactly, I hope, we will be able to get very exact solution of the above type of mean reverting SDEs.
As I am trying to complete the program for monte carlo simulation of explicit time dependent SDEs, once I post that program in a few days, I will come back to integrating the above solution dynamics in our Ito-hermite framework for the solution of above mean reverting SDEs.
I have used the above technique to solve the mean reverting SDEs in Ito-hermite density simulation method framework. It is an improvement over the previous program. I will make further improvements in the update equation in a few days.
The program below will work only with mean-reverting type SDEs where the method of multiplication with the exponential as given in the quoted post is applicable. I have also used mean-correction.
Please note that mu1=kappa*theta, beta1=0 and mu2=-kappa, beta2=1.0 in our mean reverting type SDEs. For the above mean-reverting type
sdes. Since there is time dependence in the above method, I calculated the integrals with exponentials in them.
Here is the  new code.
function [TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected08Exponential(x0,theta,kappa,gamma,sigma0,T)

%Copyright Ahsan Amin. Infiniti derivatives Technologies.
%or skype ahsan.amin2999
%In this program, I am simulating the SDE given as
%dx(t)=kappa*(theta dt -x dt) +sigma x(t)^gamma dz(t)

%I have not directly simulated the SDE but simulated the transformed
%Besse1l process version of the SDE and then changed coordinates to retreive
%the SDE in original coo
%rdinates.
%The present program will analytically evolve only the Bessel Process version of the
%SDE in transformed coordinates.

dt=.125/16;   % Simulation time interval.%Fodiffusions close to zero
%decrease dt for accuracy.
%T=5;
Tt=128*T;%16*16;     % Number of simulation levels. Terminal time= Tt*dt; //.125/32*32*16=2 year;
OrderA=4;  %
OrderM=4;  %
%dtM=.125/8;%Monte carlo time interval size dtM.
%TtM=8*8;%Monte carlo number of simulation intervals.
dtM=dt*2;
TtM=Tt/2;

dNn=.2/1;   % Normal density subdivisions width. would change with number of subdivisions
Nn=48;  % No of normal density subdivisions
NnMidl=25;%One half density Subdivision left from mid of normal density(low)
NnMidh=26;%One half density subdivision right from the mid of normal density(high)
NnMid=4.0;

%theta=mu1/(-mu2);
%kappa=-mu2;

%x0=.25;   % starting value of SDE
beta1=0.0;
beta2=1.0;   % Second drift term power.
%gamma=.95;%50;   % volatility power.
%kappa=.5;   %mean reversion parameter.
%theta=.075;%mean reversion target

%you can specify any general mu1 and mu2 and beta1 and beta2.
%The present program is not perfect but quite reasonably good.
%if there is only one term in drift with beta=0, there will be minor
%errors. This case is yet not covered.

mu1=+1*theta*kappa;   %first drift coefficient.
mu2=-1*kappa;    % Second drift coefficient.
%sigma0=1.0;%Volatility value
alpha=1;% x^alpha is being expanded. This is currently for monte carlo only.
alpha1=1-gamma;

w(1:Nn)=x0^(1-gamma)/(1-gamma);

Z(1:Nn)=(((1:Nn)-5.5)*dNn-NnMid);

Z
str=input('Look at Z');
ZProb(1)=normcdf(.5*Z(1)+.5*Z(2),0,1)-normcdf(.5*Z(1)+.5*Z(2)-dNn,0,1);
ZProb(Nn)=normcdf(.5*Z(Nn)+.5*Z(Nn-1)+dNn,0,1)-normcdf(.5*Z(Nn)+.5*Z(Nn-1),0,1);

for nn=2:Nn-1
ZProb(nn)=normcdf(.5*Z(nn)+.5*Z(nn+1),0,1)-normcdf(.5*Z(nn)+.5*Z(nn-1),0,1);
%Above calculate probability mass in each probability subdivision.
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

sigma11(1:OrderA+1)=0;
mu11(1:OrderA+1)=0;
mu22(1:OrderA+1)=0;
sigma22(1:OrderA+1)=0;
% index 1 correponds to zero level since matlab indexing starts at one.
sigma11(1)=1;
mu11(1)=1;
mu22(1)=1;
sigma22(1)=1;

for k=1:(OrderA+1)
if sigma0~=0
sigma11(k)=sigma0^(k-1);
end
if mu1 ~= 0
mu11(k)=mu1^(k-1);
end
if mu2 ~= 0
mu22(k)=mu2^(k-1);
end
if sigma0~=0
sigma22(k)=sigma0^(2*(k-1));
end
end
%Ft(1:TtM+1,1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0; %General time powers on hermite polynomials
Fp(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers on coefficients of hermite polynomials.
Fp1(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers for bessel transformed coordinates.

%YCoeff0 and YCoeff are coefficents for original coordinates monte carlo.
%YqCoeff0 and YqCoeff are bessel/lamperti version monte carlo.

YCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
YqCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
%Pre-compute the time and power exponent values in small multi-dimensional arrays
YCoeff = ItoTaylorCoeffsNew(alpha,beta1,beta2,gamma); %expand y^alpha where alpha=1;
YqCoeff = ItoTaylorCoeffsNew(alpha1,beta1,beta2,gamma);%expand y^alpha1 where alpha1=(1-gamma)
YqCoeff=YqCoeff/(1-gamma); %Transformed coordinates coefficients have to be
%further divided by (1-gamma)
YqCoeffExp= ItoTaylorCoeffsNewO2Exp(alpha1,beta1,beta2,gamma);
YqCoeffExp=YqCoeffExp/(1-gamma);

Order2=2;
for k = 0 : (Order2)
for m = 0:k
l4 = k - m + 1;
for n = 0 : m
l3 = m - n + 1;
for j = 0:n
l2 = n - j + 1;
l1 = j + 1;
Fp1Exp(l1,l2,l3,l4) = (alpha1 + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));
YqCoeffExp0(l1,l2,l3,l4) =YqCoeffExp(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
end
end
end
end

for k = 0 : (OrderA)
for m = 0:k
l4 = k - m + 1;
for n = 0 : m
l3 = m - n + 1;
for j = 0:n
l2 = n - j + 1;
l1 = j + 1;
%Ft(l1,l2,l3,l4) = dtM^((l1-1) + (l2-1) + (l3-1) + .5* (l4-1));
Fp(l1,l2,l3,l4) = (alpha + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));
Fp1(l1,l2,l3,l4) = (alpha1 + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));

YCoeff0(l1,l2,l3,l4) =YCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
YqCoeff0(l1,l2,l3,l4) =YqCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);

end
end
end
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
wnStart=1;%
d2wdZ2(1:Nn)=0;
dwdZ(1:Nn)=0;
yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
tic

for tt=1:Tt

kappa2=kappa*(1-gamma);
yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

%    [wMu0dt1,dwMu0dtdw1,d2wMu0dtdw21,d3wMu0dtdw31,c11,c21,c31,c41] = CalculateDriftAndVolOrder4(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt);

[wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,c1,c2,c3,c4] = CalculateDriftAndVolOrderLowExponential(w,wnStart,Nn,YqCoeffExp0,Fp1Exp,gamma,dt,kappa2);

dw(wnStart:Nn)=c1(wnStart:Nn).*Z(wnStart:Nn) ;% ...
dw2(wnStart:Nn)=dw(wnStart:Nn).^2;
% d2w(wnStart:Nn)=c2(wnStart:Nn).*(Z(wnStart:Nn).^2-1);
% d3w(wnStart:Nn)=c3(wnStart:Nn).*(Z(wnStart:Nn).^3-3*Z(wnStart:Nn));
w(isnan(w)==1)=0;
wMu0dt(isnan(wMu0dt)==1)=0;
wMeanPrev=sum(ZProb(wnStart:Nn).*w(wnStart:Nn));%Calculate the mean.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

[w0,Sigma00,Sigma11,Sigma22,Sigma33,Sigma1,Sigma2,Sigma3,Sigma0] = CalculateHermiteRepresentationOfDensityOrderThreeNew(w,Z,wnStart,Nn,dNn,NnMidh,NnMidl);
d2wdZ2(wnStart:Nn)=d2wdZ2(wnStart:Nn)+ d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2+dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn);%+ ...

dwdZ(wnStart:Nn)=(Sigma00)+Sigma1(wnStart:Nn).*2.*Z(wnStart:Nn);%.*ExpDamp(wnStart:Nn);

H2(wnStart:Nn)=(Z(wnStart:Nn).^2-1);
CTerm(wnStart:Nn)=(.5*d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2 - ...
1*dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn));
Correction1(wnStart:Nn)=-sqrt(2/3)/(22/7)^2.*CTerm(wnStart:Nn).*H2(wnStart:Nn);
Correction0(wnStart:Nn)=-sqrt(2/3)/(22/7)^2.*(d2wdZ2(wnStart:Nn)-.25*dwdZ(wnStart:Nn).^2).*H2(wnStart:Nn);

if(tt==1)
w(wnStart:Nn)=wMeanPrev+exp(-kappa2*dt).*(wMu0dt(wnStart:Nn)+ ...
sign(exp(kappa2*dt).*(w(wnStart:Nn)-wMeanPrev)+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*exp(2*kappa2*dt).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn))));
end
if(tt>1)

w(wnStart:Nn)=wMeanPrev+exp(-kappa2*dt).*(wMu0dt(wnStart:Nn)+ ...
sign(exp(kappa2*dt).*(w(wnStart:Nn)-wMeanPrev)+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*exp(2*kappa2*dt).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn))))+ ...
+dt *Correction0(wnStart:Nn).*exp(-.5*(1-gamma)*sigma0.^2.*Z(wnStart:Nn).^2) + ...
Correction1(wnStart:Nn).*exp(-.5*(1-gamma)*sigma0.^2.*Z(wnStart:Nn).^2);

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Below are instability correction equations. There is instability in the
% SDE representation equation roughly between -.5 SD to +.5 SD in normal terms.
%Instability correction Algorithm takes two ends that are here given as
%lower end=w(NnMidl-2) and upper end=w(NnMidh+3).
%In the block below the interpolation starts from lower end.
%1st equation is
%And so on.
%There is a sixth equation that is not written below(since it takes us to
%the upper interpolation point which is already known) which is
%Sum of these six equations add up to the distance between upper and lower
%interpolation points.
%In the sum there are six instances of first derivative which is already
%calculated at the boundary, six instances of second derivative which is also
%already calculated at lower boundary and 18.0 instances of DeltaD1w which is
%unknown. This equation calculates the value of one unit of DeltaD1w addition
% I hope to find a better analytical method to tackle this issue. Friends
% are welcome to play with slightly changed related alternatives to the
%above interpolation.
CTheta=1.0;
D1wl=(11/6*w(NnMidl-2)-3*w(NnMidl-3)+3/2*w(NnMidl-4)-1/3*w(NnMidl-5))/dNn;
D2wl=(35/12*w(NnMidl-2) -26/3*w(NnMidl-3)+ 19/2*w(NnMidl-4)-14/3*w(NnMidl-5)+11/12*w(NnMidl-6))/dNn.^2;

%The last equation is not used since point w(NnMidh+3)is given but it is used in
%calculation of distance between two interpolation points. This distance is
%used to determine the value of DeltaD1w.

D1wh=(-11/6*w(NnMidh+3)+3*w(NnMidh+4)-3/2*w(NnMidh+5)+1/3*w(NnMidh+6))/dNn;
D2wh=(35/12*w(NnMidh+3) -26/3*w(NnMidh+4)+ 19/2*w(NnMidh+5)-14/3*w(NnMidh+6)+11/12*w(NnMidh+7))/dNn.^2;

%
w(NnMidh+2)=.5*whh(NnMidh+2)+.5*wll(NnMidh+2);
w(NnMidh+1)=.5*whh(NnMidh+1)+.5*wll(NnMidh+1);
w(NnMidh)=.5*whh(NnMidh)+.5*wll(NnMidh);
w(NnMidl)=.5*whh(NnMidl)+.5*wll(NnMidl);
w(NnMidl-1)=.5*whh(NnMidl-1)+.5*wll(NnMidl-1);

w1(1:Nn-1)=w(1:Nn-1);
w2(1:Nn-1)=w(2:Nn);

%         w1(1:Nn-2)=w(1:Nn-2);
%         w2(1:Nn-2)=w(2:Nn-1);
%         w3(1:Nn-2)=w(3:Nn);
%         w(w1(:)>w2(:))=w2(w1(:)>w2(:))-.25*(w3(w1(:)>w2(:))-w2(w1(:)>w2(:)));
w(w1(:)>w2(:))=0;%Be careful;might not universally hold;

w(w<0)=0.0;
for nn=1:Nn
if(w(nn)<=0)
wnStart=nn+1;
end
end

%%1st order mean correction. We know the mean in original
%%coordinates so I shift the density into original coordinates,
%%apply the mean correction and then transform again into Lamperti
%%coordinates. Algebra solves two equations in two unknowns.
%%Equations are Sum_0^N{(Y_w(wnStart:Nn)-Y0)*W0}=1 and the second
%%equation is Sum_0^N{Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0}=u0
%%Two unknows are Y0 and W0. u0 is known mean.
tt;
u0=theta+(x0-theta)*exp(-kappa*(tt*dt)); %analytic mean of the density

%If you are not using stochastic volatility, replace above with
%true mean otherwise results would become garbage.

Y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

Yn2Pn=sum(Y_w(wnStart:Nn).*Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
YnPn=sum(Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
Pn=1.0;%%Sum(ZProb(1:Nn))
Y0=(Yn2Pn-u0*YnPn)/(YnPn-u0*Pn);

Y0Pn=Y0*Pn;
W0=1/(YnPn-Y0Pn);
YCorrected(wnStart:Nn)=Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0;
wCorrected(wnStart:Nn)=(YCorrected(wnStart:Nn).^(1-gamma))./(1-gamma);
w(wnStart:Nn)=wCorrected(wnStart:Nn);

%I have disabled the mean correction. The above mean correction is only valid for
%standard mean reverting stochastic volatility type SDEs. To enable mean
%correction please uncomment the above last line in the block.

end

y_w(1:Nn)=0;
y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
Dfy_w(wnStart:Nn)=0;
Dfw(wnStart:Nn)=0;
for nn=wnStart+1:Nn-1
Dfy_w(nn) = (y_w(nn + 1) - y_w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
Dfw(nn) = (w(nn + 1) - w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
%Change of variable derivative for densities
end
fy_w(1:Nn)=0;
fw(1:Nn)=0;
for nn = wnStart:Nn-1
fy_w(nn) = (normpdf(Z(nn),0, 1))/abs(Dfy_w(nn));%Origianl coordinates density
fw(nn) = (normpdf(Z(nn),0, 1))/abs(Dfw(nn));
end

toc
%str=input('Analytic Values calculated; Press a key to start monte carlo');

rng(29079137, 'twister')
paths=100000;
YY(1:paths)=x0;  %Original process monte carlo.
Random1(1:paths)=0;
YYMean(1:TtM)=0;
for tt=1:TtM
Random1=randn(size(Random1));
HermiteP1(1,1:paths)=1;
HermiteP1(2,1:paths)=Random1(1:paths);
HermiteP1(3,1:paths)=Random1(1:paths).^2-1;
HermiteP1(4,1:paths)=Random1(1:paths).^3-3*Random1(1:paths);
HermiteP1(5,1:paths)=Random1(1:paths).^4-6*Random1(1:paths).^2+3;

YY(1:paths)=YY(1:paths) + ...
(YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+ ...
YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
(YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+ ...
YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+ ...
YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+ ...
YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
(YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+ ...
YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5) .*HermiteP1(2,1:paths) + ...
((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
(YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+ ...
YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2).*HermiteP1(3,1:paths) + ...
((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )).*HermiteP1(4,1:paths) + ...
(YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);

%Uncomment for fourth order monte carlo

%
%   YY(1:paths)=YY(1:paths) + ...
%       (YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
%       YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
%       (YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
%       YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
%       YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
%       (YCoeff0(1,1,4,1).*YY(1:paths).^Fp(1,1,4,1)+YCoeff0(1,2,3,1).*YY(1:paths).^Fp(1,2,3,1)+ ...
%       YCoeff0(2,1,3,1).*YY(1:paths).^Fp(2,1,3,1)+YCoeff0(1,3,2,1).*YY(1:paths).^Fp(1,3,2,1)+ ...
%       YCoeff0(2,2,2,1).*YY(1:paths).^Fp(2,2,2,1)+YCoeff0(3,1,2,1).*YY(1:paths).^Fp(3,1,2,1)+ ...
%       YCoeff0(1,4,1,1).*YY(1:paths).^Fp(1,4,1,1)+YCoeff0(2,3,1,1).*YY(1:paths).^Fp(2,3,1,1)+ ...
%       YCoeff0(3,2,1,1).*YY(1:paths).^Fp(3,2,1,1)+YCoeff0(4,1,1,1).*YY(1:paths).^Fp(4,1,1,1))*dtM^3 + ...
%        (YCoeff0(1,1,5,1).*YY(1:paths).^Fp(1,1,5,1)+YCoeff0(1,2,4,1).*yy(1:paths).^Fp(1,2,4,1)+ ...
%     YCoeff0(2,1,4,1).*YY(1:paths).^Fp(2,1,4,1)+YCoeff0(2,1,1,1).*yy(1:paths).^Fp(2,1,1,1)+ ...
%      YCoeff0(2,2,3,1).*YY(1:paths).^Fp(2,2,3,1)+ ...
%      YCoeff0(3,1,3,1).*YY(1:paths).^Fp(3,1,3,1)+YCoeff0(1,4,2,1).*YY(1:paths).^Fp(1,4,2,1)+ ...
%      YCoeff0(2,3,2,1).*YY(1:paths).^Fp(2,3,2,1)+YCoeff0(3,2,2,1).*YY(1:paths).^Fp(3,2,2,1)+ ...
%      YCoeff0(4,1,2,1).*YY(1:paths).^Fp(4,1,2,1)+YCoeff0(1,5,1,1).*YY(1:paths).^Fp(1,5,1,1)+ ...
%      YCoeff0(2,4,1,1).*YY(1:paths).^Fp(2,4,1,1)+ ...
%      YCoeff0(3,3,1,1).*YY(1:paths).^Fp(3,3,1,1)+YCoeff0(4,2,1,1).*YY(1:paths).^Fp(4,2,1,1)+ ...
%      YCoeff0(5,1,1,1).*YY(1:paths).^Fp(5,1,1,1))*dtM^4+ ...
%       ((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
%     (YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
%     YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5+ ...
%     (YCoeff0(1,1,3,2).*YY(1:paths).^Fp(1,1,3,2)+YCoeff0(1,2,2,2).*YY(1:paths).^Fp(1,2,2,2)+ ...
%     YCoeff0(2,1,2,2).*YY(1:paths).^Fp(2,1,2,2)+YCoeff0(1,3,1,2).*YY(1:paths).^Fp(1,3,1,2)+ ...
%     YCoeff0(2,2,1,2).*YY(1:paths).^Fp(2,2,1,2)+YCoeff0(3,1,1,2).*YY(1:paths).^Fp(3,1,1,2)).*dtM^2.5+ ...
%     (YCoeff0(1,1,4,2).*YY(1:paths).^Fp(1,1,4,2)+YCoeff0(1,2,3,2).*YY(1:paths).^Fp(1,2,3,2)+ ...
%     YCoeff0(2,1,3,2).*YY(1:paths).^Fp(2,1,3,2)+YCoeff0(1,3,2,2).*YY(1:paths).^Fp(1,3,2,2)+ ...
%     YCoeff0(2,2,2,2).*YY(1:paths).^Fp(2,2,2,2)+ YCoeff0(3,1,2,2).*YY(1:paths).^Fp(3,1,2,2)+ ...
%     YCoeff0(1,4,1,2).*YY(1:paths).^Fp(1,4,1,2)+YCoeff0(2,3,1,2).*YY(1:paths).^Fp(2,3,1,2)+ ...
%     YCoeff0(3,2,1,2).*YY(1:paths).^Fp(3,2,1,2)+YCoeff0(4,1,1,2).*YY(1:paths).^Fp(4,1,1,2)).*dtM^3.5) .*HermiteP1(2,1:paths) + ...
%     ((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
%     (YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
%     YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2+ ...
%     (YCoeff0(1,1,3,3).*YY(1:paths).^Fp(1,1,3,3)+YCoeff0(1,2,2,3).*YY(1:paths).^Fp(1,2,2,3)+ ...
%     YCoeff0(2,1,2,3).*YY(1:paths).^Fp(2,1,2,3) + YCoeff0(1,3,1,3).*YY(1:paths).^Fp(1,3,1,3)+ ...
%     YCoeff0(2,2,1,3).*YY(1:paths).^Fp(2,2,1,3)+YCoeff0(3,1,1,3).*YY(1:paths).^Fp(3,1,1,3)).*dtM^3).*HermiteP1(3,1:paths) + ...
%     ((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )+ ...
%     (YCoeff0(1,1,2,4).*YY(1:paths).^Fp(1,1,2,4)+YCoeff0(1,2,1,4).*YY(1:paths).^Fp(1,2,1,4)+ ...
%     YCoeff0(2,1,1,4).*YY(1:paths).^Fp(2,1,1,4))*dtM^2.5).*HermiteP1(4,1:paths) + ...
%     (YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);
%

YYMean(tt)=YYMean(tt)+sum(YY(1:paths))/paths;
end
YY(YY<0)=0;
disp('Original process average from monte carlo');
MCMean=sum(YY(:))/paths %origianl coordinates monte carlo average.
disp('Original process average from our simulation');
ItoHermiteMean=sum(y_w(wnStart+1:Nn-1).*ZProb(wnStart+1:Nn-1)) %Original process average from coordinates

disp('true Mean only applicble to standard SV mean reverting type models otherwise disregard');
TrueMean=theta+(x0-theta)*exp(-kappa*dt*Tt)%Mean reverting SDE original variable true average

MaxCutOff=30;
NoOfBins=round(500*gamma^2*4*sigma0);
[YDensity,IndexOutY,IndexMaxY] = MakeDensityFromSimulation_Infiniti_NEW(YY,paths,NoOfBins,MaxCutOff );
plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g');
%plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g',Z(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'b');

title(sprintf('Ito-Hermite Method VS Monte Carlo Density, x0 = %.2f,theta=%.2f,kappa=%.2f,gamma=%.3f,sigma=%.2f,T=%.2f', x0,theta,kappa,gamma,sigma0,T));%,sprintf('theta= %f', theta), sprintf('kappa = %f', kappa),sprintf('sigma = %f', sigma0),sprintf('T = %f', T));

%title('text',sprintf('x0 = %f', x0),sprintf('theta= %f', theta), sprintf('kappa = %f', kappa),sprintf('sigma = %f', sigma0),sprintf('T = %f', T))
%xlabel(a)
%title('Graphical Comparison of Ito-Hermite Method and Monte Carlo Method Density')
legend({'Ito-Hermite Density','Monte Carlo Density'},'Location','northeast')

str=input('red line is density of SDE from Ito-Hermite method, green is monte carlo.');

end



Here is the first dependeny of the above program
function [Y] = ItoTaylorCoeffsNewO2Exp(alpha,beta1,beta2,gamma)

%In the coefficient calculation program which calculates Y(l1,l2,l3,l4),
%I have used four levels of looping each for relevant expansion order.
%The first loop takes four values and second loop takes 16 values and
%third loop takes 64 values and so on. And then each coefficient
%term can be individually calculated while carefully accounting
%for path dependence.
%So for example in a nested loop structure
%m1= 1:mDim
% m2=1:mDim
% m3=1:mDim
%    l(m1)=l(m1)+1;
%    l(m2)=l(m2)+1;
%    l(m3)=l(m3)+1;

%in the above looping loop takes values from one to four with one
%indicating the first drift term, two indicating the second drift term
%and three indicating quadratic variation term and
%four indicating the volatility term. And with this looping structure
%we can So in the above looping m1=1 would mean that all terms are
%descendent of first drift term and m2=4 would mean that all terms are
%descendent of first drift term on first expansion order and descendent
%of volatility term on the second order and so we can keep track of path
%dependence perfectly.
%And on each level, we individually calculate the descendent terms. While
%keeping track of path dependence and calculating the coefficients with
%careful path dependence consideration, we update the appropriate element
%in our polynomial like expansion coefficient array

%explaining the part of code
%m1= 1:mDim
% m2=1:mDim
% m3=1:mDim
%    l(m1)=l(m1)+1;
%    l(m2)=l(m2)+1;
%    l(m3)=l(m3)+1;
%Y(l(1)+1,l(2),l(3),l(4))=Y(l(1)+1,l(2),l(3),l(4))+Coeff1st*IntegralCoeff(1,1,1,2);

%Here l(1) denotes l1 but written as l(1) so it can be conveniently
%updated with the loop variable when the loop variable takes value one
%indicating first drift term . And l(2) could be conveniently updated when
%the loop variable takes value equal to two indicating second
%drift term and so on.
%Here is the part of code snippet for that

%for m1=1:mDim
%    l(1)=1;
%    l(2)=1;
%    l(3)=1;
%    l(4)=1;
%    l(m1)=l(m1)+1;
%CoeffDX1 = alpha + (l(1)-1) *beta1 + (l(2)-1) *beta2 + (l(3)-1) *2*gamma + (l(4)-1)*gamma - (l(1)-1) - (l(2)-1) - 2*(l(3)-1) - (l(4)-1);
%    CoeffDX2 = CoeffDX1 - 1;
%    ArrIndex0=m1;
%    ArrIndex=(m1-1)*mDim;
%    Coeff1st=Y1(ArrIndex0)*CoeffDX1;
%    Coeff2nd=Y1(ArrIndex0)*.5*CoeffDX1*CoeffDX2;
%    Y2(1+ArrIndex)=Coeff1st;
%    Y(l(1)+1,l(2),l(3),l(4))=Y(l(1)+1,l(2),l(3),l(4))+Coeff1st*IntegralCoeff(1,1,2,n1(m1));
%    Y2(2+ArrIndex)=Coeff1st;
%    Y(l(1),l(2)+1,l(3),l(4))=Y(l(1),l(2)+1,l(3),l(4))+Coeff1st*IntegralCoeff(1,1,2,n1(m1));
%    Y2(3+ArrIndex)=Coeff2nd;
%    Y(l(1),l(2),l(3)+1,l(4))=Y(l(1),l(2),l(3)+1,l(4))+Coeff2nd*IntegralCoeff(1,1,2,n1(m1));
%    Y2(4+ArrIndex)=Coeff1st;
%    Y(l(1),l(2),l(3),l(4)+1)=Y(l(1),l(2),l(3),l(4)+1)+Coeff1st*IntegralCoeff(1,1,3,n1(m1));

%The first four lines update the array indices according to the parent term.
%And then CoeffDX1 and CoeffDX2 are calculated according to algebraic exponents on parent terms.
%ArrIndex0=m1; calculates the array index of the parent term
%And ArrIndex=(m1-1)*mDim; calculates the array index of the descendent terms
%And coefficient of the drift and volatility descendent terms is
%calculated by multiplying the coefficient of the parent term by
%Coeff1st=Y1(ArrIndex0)*CoeffDX1;
%And coefficient of the quadratic variation descendent terms is
%calculated by multiplying the coefficient of the parent term by
%Coeff2nd=Y1(ArrIndex0)*.5*CoeffDX1*CoeffDX2;
%And then each of the four descendent terms are updated with Coeff1st
%if they are drift or volatility descendent terms or Coeff2nd if
%they are quadratic variation descendent terms.
%Here Y1 indicates the temporary coefficient array with parent terms on
%first level. And Y2 denotes temporary coefficient array with parent terms
%on second level and Y3 indicates temporary coefficient array with parent terms on third level.

%[IntegralCoeff,IntegralCoeffdt,IntegralCoeffdz] = ComputeIntegralCoeffs();

n1(1)=2;
n1(2)=2;
n1(3)=2;
n1(4)=3;
%n1(1), n1(2), n1(3) are drift and quadratic variation term variables
%and take a value equal to 2 indicating a dt integral.
%n1(4) is volatility term variable and indicates a dz-integral by taking a
%value of 3.

mDim=4; % four descendent terms in each expansion
Y(1:3,1:3,1:3,1:3)=0;
Y1(1:mDim)=0;
%Y2(1:mDim*mDim)=0;
%Y3(1:mDim*mDim*mDim)=0;

%First Ito-hermite expansion level starts here. No loops but four
%descendent terms.
l(1)=1;
l(2)=1;
l(3)=1;
l(4)=1;
CoeffDX1 = alpha;
CoeffDX2 = CoeffDX1 - 1;
Coeff1st=CoeffDX1;
Coeff2nd=.5*CoeffDX1*CoeffDX2;

Y1(1)=Coeff1st;
Y(l(1)+1,l(2),l(3),l(4))=Y(l(1)+1,l(2),l(3),l(4))+Coeff1st;
Y1(2)=Coeff1st;
Y(l(1),l(2)+1,l(3),l(4))=Y(l(1),l(2)+1,l(3),l(4))+Coeff1st;
Y1(3)=Coeff2nd;
Y(l(1),l(2),l(3)+1,l(4))=Y(l(1),l(2),l(3)+1,l(4))+Coeff2nd;
Y1(4)=Coeff1st;
Y(l(1),l(2),l(3),l(4)+1)=Y(l(1),l(2),l(3),l(4)+1)+Coeff1st;

%Second Ito-hermite expansion level starts. It has a loop over four parent
%terms and there are four descendent terms for each parent term.
%The coefficient terms are then lumped in a polynomial-like expansion
%array of coefficients.
for m1=1:mDim
l(1)=1;
l(2)=1;
l(3)=1;
l(4)=1;
l(m1)=l(m1)+1;
CoeffDX1 = alpha + (l(1)-1) *beta1 + (l(2)-1) *beta2 + (l(3)-1) *2*gamma + (l(4)-1)*gamma ...
- (l(1)-1) - (l(2)-1) - 2*(l(3)-1) - (l(4)-1);
CoeffDX2 = CoeffDX1 - 1;
ArrIndex0=m1;

Coeff1st=Y1(ArrIndex0)*CoeffDX1;
Coeff2nd=Y1(ArrIndex0)*.5*CoeffDX1*CoeffDX2;

Y(l(1)+1,l(2),l(3),l(4))=Y(l(1)+1,l(2),l(3),l(4))+Coeff1st;
Y(l(1),l(2)+1,l(3),l(4))=Y(l(1),l(2)+1,l(3),l(4))+Coeff1st;
Y(l(1),l(2),l(3)+1,l(4))=Y(l(1),l(2),l(3)+1,l(4))+Coeff2nd;
Y(l(1),l(2),l(3),l(4)+1)=Y(l(1),l(2),l(3),l(4)+1)+Coeff1st;
end
end
Please run the program on command line with the following command line arguments
[TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected08Exponential(.10,.10,1,.75,.9,3)
and expect the output as
ItoHermiteMean =0.099992643048797;
MCMean =0.099441733129894
Last edited by Amin on March 29th, 2020, 9:20 pm, edited 1 time in total.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

Sorry one of the sub-functions required to run the main program that I posted above had a slight error. I am posting it again here and I am removing it from previous post.
function [wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,c1,c2,c3,c4] = CalculateDriftAndVolOrderLowExponential(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt,kappa)

yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

Fp2=Fp1/(1-gamma);

dt1=(exp(kappa*dt)-1)/kappa;
dt2=(1 + exp(kappa* dt)* (-1 + kappa* dt))/kappa^2;

wMu0dt(wnStart:Nn)=(YqCoeff0(1,1,2,1).*yy(wnStart:Nn).^Fp1(1,1,2,1)+ ...
YqCoeff0(1,2,1,1).*yy(wnStart:Nn).^Fp1(1,2,1,1)+ ...
YqCoeff0(2,1,1,1).*yy(wnStart:Nn).^Fp1(2,1,1,1))*dt1 + ...
(YqCoeff0(1,1,3,1).*yy(wnStart:Nn).^Fp1(1,1,3,1)+ ...
YqCoeff0(1,2,2,1).*yy(wnStart:Nn).^Fp1(1,2,2,1)+ ...
YqCoeff0(2,1,2,1).*yy(wnStart:Nn).^Fp1(2,1,2,1)+ ...
YqCoeff0(1,3,1,1).*yy(wnStart:Nn).^Fp1(1,3,1,1)+ ...
YqCoeff0(2,2,1,1).*yy(wnStart:Nn).^Fp1(2,2,1,1)+ ...
YqCoeff0(3,1,1,1).*yy(wnStart:Nn).^Fp1(3,1,1,1))*dt2;
%     (YqCoeff0(1,1,4,1).*yy(wnStart:Nn).^Fp1(1,1,4,1)+ ...
%     YqCoeff0(1,2,3,1).*yy(wnStart:Nn).^Fp1(1,2,3,1)+ ...
%     YqCoeff0(2,1,3,1).*yy(wnStart:Nn).^Fp1(2,1,3,1)+ ...
%     YqCoeff0(1,3,2,1).*yy(wnStart:Nn).^Fp1(1,3,2,1)+ ...
%     YqCoeff0(2,2,2,1).*yy(wnStart:Nn).^Fp1(2,2,2,1)+ ...
%     YqCoeff0(3,1,2,1).*yy(wnStart:Nn).^Fp1(3,1,2,1)+ ...
%     YqCoeff0(1,4,1,1).*yy(wnStart:Nn).^Fp1(1,4,1,1)+ ...
%     YqCoeff0(2,3,1,1).*yy(wnStart:Nn).^Fp1(2,3,1,1)+ ...
%     YqCoeff0(3,2,1,1).*yy(wnStart:Nn).^Fp1(3,2,1,1)+ ...
%     YqCoeff0(4,1,1,1).*yy(wnStart:Nn).^Fp1(4,1,1,1))*dt^3;

dwMu0dtdw(wnStart:Nn)=(YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,1,1)))*dt1 + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,1,1,1)))*dt2;% + ...
%     (YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,4,1))+ ...
%     YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,3,1))+ ...
%     YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,3,1))+ ...
%     YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,3,2,1))+ ...
%     YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,2,2,1))+ ...
%     YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,1,2,1))+ ...
%     YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,4,1,1))+ ...
%     YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,3,1,1))+ ...
%     YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,2,1,1))+ ...
%     YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(4,1,1,1)))*dt^3;

d2wMu0dtdw2(wnStart:Nn)=(1-gamma).*((YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*(-1+Fp2(1,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*(-1+Fp2(1,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*(-1+Fp2(2,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,1,1)))*dt1 + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*(-1+Fp2(1,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*(-1+Fp2(1,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*(-1+Fp2(2,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*(-1+Fp2(1,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*(-1+Fp2(2,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*(-1+Fp2(3,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,1,1,1)))*dt2);% + ...
%     (YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*(-1+Fp2(1,1,4,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,4,1))+ ...
%     YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*(-1+Fp2(1,2,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,3,1))+ ...
%     YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*(-1+Fp2(2,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,3,1))+ ...
%     YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*(-1+Fp2(1,3,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,3,2,1))+ ...
%     YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*(-1+Fp2(2,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,2,2,1))+ ...
%     YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*(-1+Fp2(3,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,1,2,1))+ ...
%     YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*(-1+Fp2(1,4,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,4,1,1))+ ...
%     YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*(-1+Fp2(2,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,3,1,1))+ ...
%     YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*(-1+Fp2(3,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,2,1,1))+ ...
%     YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*(-1+Fp2(4,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(4,1,1,1)))*dt^3);
%

d3wMu0dtdw3(wnStart:Nn)=(1-gamma)^2.*((YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*(-1+Fp2(1,1,2,1)).*(-2+Fp2(1,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*(-1+Fp2(1,2,1,1)).*(-2+Fp2(1,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*(-1+Fp2(2,1,1,1)).*(-2+Fp2(2,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,1,1)))*dt1 + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*(-1+Fp2(1,1,3,1)).*(-2+Fp2(1,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*(-1+Fp2(1,2,2,1)).*(-2+Fp2(1,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*(-1+Fp2(2,1,2,1)).*(-2+Fp2(2,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*(-1+Fp2(1,3,1,1)).*(-2+Fp2(1,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*(-1+Fp2(2,2,1,1)).*(-2+Fp2(2,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*(-1+Fp2(3,1,1,1)).*(-2+Fp2(3,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,1,1,1)))*dt2);% + ...
%     (YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*(-1+Fp2(1,1,4,1)).*(-2+Fp2(1,1,4,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,4,1))+ ...
%     YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*(-1+Fp2(1,2,3,1)).*(-2+Fp2(1,2,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,3,1))+ ...
%     YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*(-1+Fp2(2,1,3,1)).*(-2+Fp2(2,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,3,1))+ ...
%     YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*(-1+Fp2(1,3,2,1)).*(-2+Fp2(1,3,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,3,2,1))+ ...
%     YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*(-1+Fp2(2,2,2,1)).*(-2+Fp2(2,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,2,2,1))+ ...
%     YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*(-1+Fp2(3,1,2,1)).*(-2+Fp2(3,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,1,2,1))+ ...
%     YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*(-1+Fp2(1,4,1,1)).*(-2+Fp2(1,4,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,4,1,1))+ ...
%     YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*(-1+Fp2(2,3,1,1)).*(-2+Fp2(2,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,3,1,1))+ ...
%     YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*(-1+Fp2(3,2,1,1)).*(-2+Fp2(3,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,2,1,1))+ ...
%     YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*(-1+Fp2(4,1,1,1)).*(-2+Fp2(4,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(4,1,1,1)))*dt^3);

%b0=1;
%b1=A/2;
%b2=5/24*A^2;
%b3=1/16*A^3;
%b4=.01371*A^4;

%d1=1.0;
%d2=.75;
%d3=.40277;
%d4=.1718;
%d5=.06106;

%ExpIntegral=dt^1.5*(.4226+.4252*kappa*dt+.2554*kappa^2*dt^2+.1178*kappa^3*dt^3+.04312*kappa^4*dt^4);
ExpIntegral=dt^1.5*((1+kappa*dt/2+kappa^2*dt^2/6+kappa^3*dt^3/24+kappa^4*dt^4/120)- ...
sqrt(1/3+kappa*dt/4+7/60*kappa^2*dt^2+1/24*kappa^3*dt^3+31/2520*kappa^4*dt^4+1/360*kappa^5*dt^5));

c1(wnStart:Nn)=YqCoeff0(1,1,1,2).*yy(wnStart:Nn).^Fp1(1,1,1,2).*sqrt((exp(2*kappa*dt)-1)/(2*kappa))+ ...
(YqCoeff0(1,1,2,2).*yy(wnStart:Nn).^Fp1(1,1,2,2)+YqCoeff0(1,2,1,2).*yy(wnStart:Nn).^Fp1(1,2,1,2)+ ...
YqCoeff0(2,1,1,2).*yy(wnStart:Nn).^Fp1(2,1,1,2)).*ExpIntegral;%+ ...
%(YqCoeff0(1,1,3,2).*yy(wnStart:Nn).^Fp1(1,1,3,2)+YqCoeff0(1,2,2,2).*yy(wnStart:Nn).^Fp1(1,2,2,2)+ ...
%YqCoeff0(2,1,2,2).*yy(wnStart:Nn).^Fp1(2,1,2,2)+YqCoeff0(1,3,1,2).*yy(wnStart:Nn).^Fp1(1,3,1,2)+ ...
%YqCoeff0(2,2,1,2).*yy(wnStart:Nn).^Fp1(2,2,1,2)+YqCoeff0(3,1,1,2).*yy(wnStart:Nn).^Fp1(3,1,1,2)).*dt^2.5;

c2(wnStart:Nn)=0.0;%YqCoeff0(1,1,1,3).*yy(wnStart:Nn).^Fp1(1,1,1,3) *dt + ...
%  (YqCoeff0(1,1,2,3).*yy(wnStart:Nn).^Fp1(1,1,2,3)+YqCoeff0(1,2,1,3).*yy(wnStart:Nn).^Fp1(1,2,1,3)+ ...
%  YqCoeff0(2,1,1,3).*yy(wnStart:Nn).^Fp1(2,1,1,3)).*dt^2+ ...
%  (YqCoeff0(1,1,3,3).*yy(wnStart:Nn).^Fp1(1,1,3,3)+YqCoeff0(1,2,2,3).*yy(wnStart:Nn).^Fp1(1,2,2,3)+ ...
%  YqCoeff0(2,1,2,3).*yy(wnStart:Nn).^Fp1(2,1,2,3) + YqCoeff0(1,3,1,3).*yy(wnStart:Nn).^Fp1(1,3,1,3)+ ...
%  YqCoeff0(2,2,1,3).*yy(wnStart:Nn).^Fp1(2,2,1,3)+YqCoeff0(3,1,1,3).*yy(wnStart:Nn).^Fp1(3,1,1,3)).*dt^3;

c3(wnStart:Nn)=0.0;%YqCoeff0(1,1,1,4).*yy(wnStart:Nn).^Fp1(1,1,1,4)*dt^1.5 + ...
%   (YqCoeff0(1,1,2,4).*yy(wnStart:Nn).^Fp1(1,1,2,4)+YqCoeff0(1,2,1,4).*yy(wnStart:Nn).^Fp1(1,2,1,4)+ ...
%   YqCoeff0(2,1,1,4).*yy(wnStart:Nn).^Fp1(2,1,1,4))*dt^2.5;

c4(wnStart:Nn)=0.0;%YqCoeff0(1,1,1,5).*yy(wnStart:Nn).^Fp1(1,1,1,5)*dt^2.0;

end
With this new function, When you run the program on command line with the following command
[TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected08Exponential(.10,.10,1,.75,.9,5)
you will get the results
ItoHermiteMean =0.099992618140914
MCMean =0.099758917684717

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

In the previous version of the program that I uploaded yesterday, there was quite a bit of improvement and the density was not getting clipped at the start but the performance of the algorithm still deteriorated when volatility increased beyond normal limits. I worked on it and improved the program and now it works quite well for reasonably high volatilities. For very high volatilities, the performance still does deteriorate as we move towards gamma=.5 but even the case of gamma=.5 is quite better than before especially away from zero. I am showing some graphs that I made with new version of the program and all of these graphs have parameters chosen to be relatively high volatility. I am posting the graphs and I will attach the new Ito-hermite program with the next post in a few minutes.

In the graph below x0=.001, and theta=.001 but I had specified only two digit precision in the graph title so that graph shows the x0=.00 and theta=.00. Since I have already uploaded the image on a hosting site, instead of making the graph again, I am making the correction about it here for the friends.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

Here is the new improved version of Ito-Hermite program for simulating the densities of SDEs. This program works only with mean reverting equations using mean-correction algorithm. But when the mean is known, the update equation of the general SDE has to be modified in a very similar manner using correction terms that I have used in this program. The correction terms in the main update equation are the same for mean-reverting SDEs and other different SDEs. Here is the main body of the program that will work with older dependent sub-functions that I have already attached with previous posts.

function [TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected08Exponential02(x0,theta,kappa,gamma,sigma0,T)

%Copyright Ahsan Amin. Infiniti derivatives Technologies.
%or skype ahsan.amin2999
%In this program, I am simulating the SDE given as
%dx(t)=kappa*(theta dt -x dt) +sigma x(t)^gamma dz(t)

%I have not directly simulated the SDE but simulated the transformed
%Besse1l process version of the SDE and then changed coordinates to retreive
%the SDE in original coo
%rdinates.
%The present program will analytically evolve only the Bessel Process version of the
%SDE in transformed coordinates.

dt=.125/16;   % Simulation time interval.%Fodiffusions close to zero
%decrease dt for accuracy.
%T=5;
Tt=128*T;%16*16;     % Number of simulation levels. Terminal time= Tt*dt; //.125/32*32*16=2 year;
OrderA=4;  %
OrderM=4;  %
%dtM=.125/8;%Monte carlo time interval size dtM.
%TtM=8*8;%Monte carlo number of simulation intervals.
dtM=dt*2;
TtM=Tt/2;

dNn=.2/1;   % Normal density subdivisions width. would change with number of subdivisions
Nn=48;  % No of normal density subdivisions
NnMidl=25;%One half density Subdivision left from mid of normal density(low)
NnMidh=26;%One half density subdivision right from the mid of normal density(high)
NnMid=4.0;

%theta=mu1/(-mu2);
%kappa=-mu2;

%x0=.25;   % starting value of SDE
beta1=0.0;
beta2=1.0;   % Second drift term power.
%gamma=.95;%50;   % volatility power.
%kappa=.5;   %mean reversion parameter.
%theta=.075;%mean reversion target

%you can specify any general mu1 and mu2 and beta1 and beta2.
%The present program is not perfect but quite reasonably good.
%if there is only one term in drift with beta=0, there will be minor
%errors. This case is yet not covered.

mu1=+1*theta*kappa;   %first drift coefficient.
mu2=-1*kappa;    % Second drift coefficient.
%sigma0=1.0;%Volatility value
alpha=1;% x^alpha is being expanded. This is currently for monte carlo only.
alpha1=1-gamma;

w(1:Nn)=x0^(1-gamma)/(1-gamma);

Z(1:Nn)=(((1:Nn)-5.5)*dNn-NnMid);

Z
str=input('Look at Z');
ZProb(1)=normcdf(.5*Z(1)+.5*Z(2),0,1)-normcdf(.5*Z(1)+.5*Z(2)-dNn,0,1);
ZProb(Nn)=normcdf(.5*Z(Nn)+.5*Z(Nn-1)+dNn,0,1)-normcdf(.5*Z(Nn)+.5*Z(Nn-1),0,1);

for nn=2:Nn-1
ZProb(nn)=normcdf(.5*Z(nn)+.5*Z(nn+1),0,1)-normcdf(.5*Z(nn)+.5*Z(nn-1),0,1);
%Above calculate probability mass in each probability subdivision.
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

sigma11(1:OrderA+1)=0;
mu11(1:OrderA+1)=0;
mu22(1:OrderA+1)=0;
sigma22(1:OrderA+1)=0;
% index 1 correponds to zero level since matlab indexing starts at one.
sigma11(1)=1;
mu11(1)=1;
mu22(1)=1;
sigma22(1)=1;

for k=1:(OrderA+1)
if sigma0~=0
sigma11(k)=sigma0^(k-1);
end
if mu1 ~= 0
mu11(k)=mu1^(k-1);
end
if mu2 ~= 0
mu22(k)=mu2^(k-1);
end
if sigma0~=0
sigma22(k)=sigma0^(2*(k-1));
end
end
%Ft(1:TtM+1,1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0; %General time powers on hermite polynomials
Fp(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers on coefficients of hermite polynomials.
Fp1(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers for bessel transformed coordinates.

%YCoeff0 and YCoeff are coefficents for original coordinates monte carlo.
%YqCoeff0 and YqCoeff are bessel/lamperti version monte carlo.

YCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
YqCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
%Pre-compute the time and power exponent values in small multi-dimensional arrays
YCoeff = ItoTaylorCoeffsNew(alpha,beta1,beta2,gamma); %expand y^alpha where alpha=1;
YqCoeff = ItoTaylorCoeffsNew(alpha1,beta1,beta2,gamma);%expand y^alpha1 where alpha1=(1-gamma)
YqCoeff=YqCoeff/(1-gamma); %Transformed coordinates coefficients have to be
%further divided by (1-gamma)
YqCoeffExp= ItoTaylorCoeffsNewO2Exp(alpha1,beta1,beta2,gamma);
YqCoeffExp=YqCoeffExp/(1-gamma);

Order2=2;
for k = 0 : (Order2)
for m = 0:k
l4 = k - m + 1;
for n = 0 : m
l3 = m - n + 1;
for j = 0:n
l2 = n - j + 1;
l1 = j + 1;
Fp1Exp(l1,l2,l3,l4) = (alpha1 + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));
YqCoeffExp0(l1,l2,l3,l4) =YqCoeffExp(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
end
end
end
end

for k = 0 : (OrderA)
for m = 0:k
l4 = k - m + 1;
for n = 0 : m
l3 = m - n + 1;
for j = 0:n
l2 = n - j + 1;
l1 = j + 1;
%Ft(l1,l2,l3,l4) = dtM^((l1-1) + (l2-1) + (l3-1) + .5* (l4-1));
Fp(l1,l2,l3,l4) = (alpha + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));
Fp1(l1,l2,l3,l4) = (alpha1 + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));

YCoeff0(l1,l2,l3,l4) =YCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
YqCoeff0(l1,l2,l3,l4) =YqCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);

end
end
end
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
wnStart=1;%
d2wdZ2(1:Nn)=0;
dwdZ(1:Nn)=0;
yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
tic

for tt=1:Tt

kappa2=kappa*(1-gamma);
yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

%    [wMu0dt1,dwMu0dtdw1,d2wMu0dtdw21,d3wMu0dtdw31,c11,c21,c31,c41] = CalculateDriftAndVolOrder4(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt);

[wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,c1,c2,c3,c4] = CalculateDriftAndVolOrderLowExponential(w,wnStart,Nn,YqCoeffExp0,Fp1Exp,gamma,dt,kappa2);

dw(wnStart:Nn)=c1(wnStart:Nn).*Z(wnStart:Nn) ;% ...
dw2(wnStart:Nn)=dw(wnStart:Nn).^2;
% d2w(wnStart:Nn)=c2(wnStart:Nn).*(Z(wnStart:Nn).^2-1);
% d3w(wnStart:Nn)=c3(wnStart:Nn).*(Z(wnStart:Nn).^3-3*Z(wnStart:Nn));
w(isnan(w)==1)=0;
wMu0dt(isnan(wMu0dt)==1)=0;
wMeanPrev=sum(ZProb(wnStart:Nn).*w(wnStart:Nn));%Calculate the mean.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

[w0,Sigma00,Sigma11,Sigma22,Sigma33,Sigma1,Sigma2,Sigma3,Sigma0] = CalculateHermiteRepresentationOfDensityOrderThreeNew(w,Z,wnStart,Nn,dNn,NnMidh,NnMidl);
d2wdZ2(wnStart:Nn)=d2wdZ2(wnStart:Nn)+ d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2+dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn);%+ ...

dwdZ(wnStart:Nn)=(Sigma00)+Sigma1(wnStart:Nn).*2.*Z(wnStart:Nn);%.*ExpDamp(wnStart:Nn);

H2(wnStart:Nn)=(Z(wnStart:Nn).^2-1);
CTerm(wnStart:Nn)=(.5*d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2 - ...
1*dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn));

Correction1(wnStart:Nn)=-(1+gamma^2.*(1-gamma)*sigma0^2)*sqrt(3/4)/(22/7)^2.*CTerm(wnStart:Nn).*H2(wnStart:Nn);
Correction0(wnStart:Nn)=-(1+gamma^2.*(1-gamma)*sigma0^2)*sqrt(3/4)/(22/7)^2.*(d2wdZ2(wnStart:Nn)-.5*dwdZ(wnStart:Nn).^2).*H2(wnStart:Nn);

if(tt==1)
w(wnStart:Nn)=wMeanPrev+exp(-kappa2*dt).*(wMu0dt(wnStart:Nn)+ ...
sign(exp(kappa2*dt).*(w(wnStart:Nn)-wMeanPrev)+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*exp(2*kappa2*dt).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn))));
end
if(tt>1)

w(wnStart:Nn)=wMeanPrev+exp(-kappa2*dt).*(wMu0dt(wnStart:Nn)+ ...
sign(exp(kappa2*dt).*(w(wnStart:Nn)-wMeanPrev)+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*exp(2*kappa2*dt).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn))))+ ...
+dt *Correction0(wnStart:Nn).*exp(-.5*(1-gamma)*sigma0.^2.*Z(wnStart:Nn).^2) + ...
Correction1(wnStart:Nn).*exp(-.5*(1-gamma)*sigma0.^2.*Z(wnStart:Nn).^2);

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Below are instability correction equations. There is instability in the
% SDE representation equation roughly between -.5 SD to +.5 SD in normal terms.
%Instability correction Algorithm takes two ends that are here given as
%lower end=w(NnMidl-2) and upper end=w(NnMidh+3).
%In the block below the interpolation starts from lower end.
%1st equation is
%And so on.
%There is a sixth equation that is not written below(since it takes us to
%the upper interpolation point which is already known) which is
%Sum of these six equations add up to the distance between upper and lower
%interpolation points.
%In the sum there are six instances of first derivative which is already
%calculated at the boundary, six instances of second derivative which is also
%already calculated at lower boundary and 18.0 instances of DeltaD1w which is
%unknown. This equation calculates the value of one unit of DeltaD1w addition
% I hope to find a better analytical method to tackle this issue. Friends
% are welcome to play with slightly changed related alternatives to the
%above interpolation.
CTheta=1.0;
D1wl=(11/6*w(NnMidl-2)-3*w(NnMidl-3)+3/2*w(NnMidl-4)-1/3*w(NnMidl-5))/dNn;
D2wl=(35/12*w(NnMidl-2) -26/3*w(NnMidl-3)+ 19/2*w(NnMidl-4)-14/3*w(NnMidl-5)+11/12*w(NnMidl-6))/dNn.^2;

%The last equation is not used since point w(NnMidh+3)is given but it is used in
%calculation of distance between two interpolation points. This distance is
%used to determine the value of DeltaD1w.

D1wh=(-11/6*w(NnMidh+3)+3*w(NnMidh+4)-3/2*w(NnMidh+5)+1/3*w(NnMidh+6))/dNn;
D2wh=(35/12*w(NnMidh+3) -26/3*w(NnMidh+4)+ 19/2*w(NnMidh+5)-14/3*w(NnMidh+6)+11/12*w(NnMidh+7))/dNn.^2;

%
w(NnMidh+2)=.5*whh(NnMidh+2)+.5*wll(NnMidh+2);
w(NnMidh+1)=.5*whh(NnMidh+1)+.5*wll(NnMidh+1);
w(NnMidh)=.5*whh(NnMidh)+.5*wll(NnMidh);
w(NnMidl)=.5*whh(NnMidl)+.5*wll(NnMidl);
w(NnMidl-1)=.5*whh(NnMidl-1)+.5*wll(NnMidl-1);

w1(1:Nn-1)=w(1:Nn-1);
w2(1:Nn-1)=w(2:Nn);

%         w1(1:Nn-2)=w(1:Nn-2);
%         w2(1:Nn-2)=w(2:Nn-1);
%         w3(1:Nn-2)=w(3:Nn);
%         w(w1(:)>w2(:))=w2(w1(:)>w2(:))-.25*(w3(w1(:)>w2(:))-w2(w1(:)>w2(:)));
w(w1(:)>w2(:))=0;%Be careful;might not universally hold;

w(w<0)=0.0;
for nn=1:Nn
if(w(nn)<=0)
wnStart=nn+1;
end
end

%%1st order mean correction. We know the mean in original
%%coordinates so I shift the density into original coordinates,
%%apply the mean correction and then transform again into Lamperti
%%coordinates. Algebra solves two equations in two unknowns.
%%Equations are Sum_0^N{(Y_w(wnStart:Nn)-Y0)*W0}=1 and the second
%%equation is Sum_0^N{Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0}=u0
%%Two unknows are Y0 and W0. u0 is known mean.
tt;
u0=theta+(x0-theta)*exp(-kappa*(tt*dt)); %analytic mean of the density

%If you are not using stochastic volatility, replace above with
%true mean otherwise results would become garbage.

Y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

Yn2Pn=sum(Y_w(wnStart:Nn).*Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
YnPn=sum(Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
Pn=1.0;%%Sum(ZProb(1:Nn))
Y0=(Yn2Pn-u0*YnPn)/(YnPn-u0*Pn);

Y0Pn=Y0*Pn;
W0=1/(YnPn-Y0Pn);
YCorrected(wnStart:Nn)=Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0;
wCorrected(wnStart:Nn)=(YCorrected(wnStart:Nn).^(1-gamma))./(1-gamma);
w(wnStart:Nn)=wCorrected(wnStart:Nn);

%I have disabled the mean correction. The above mean correction is only valid for
%standard mean reverting stochastic volatility type SDEs. To enable mean
%correction please uncomment the above last line in the block.

end

y_w(1:Nn)=0;
y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
Dfy_w(wnStart:Nn)=0;
Dfw(wnStart:Nn)=0;
for nn=wnStart+1:Nn-1
Dfy_w(nn) = (y_w(nn + 1) - y_w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
Dfw(nn) = (w(nn + 1) - w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
%Change of variable derivative for densities
end
fy_w(1:Nn)=0;
fw(1:Nn)=0;
for nn = wnStart:Nn-1
fy_w(nn) = (normpdf(Z(nn),0, 1))/abs(Dfy_w(nn));%Origianl coordinates density
fw(nn) = (normpdf(Z(nn),0, 1))/abs(Dfw(nn));
end

toc
%str=input('Analytic Values calculated; Press a key to start monte carlo');

rng(29079137, 'twister')
paths=100000;
YY(1:paths)=x0;  %Original process monte carlo.
Random1(1:paths)=0;
YYMean(1:TtM)=0;
for tt=1:TtM
Random1=randn(size(Random1));
HermiteP1(1,1:paths)=1;
HermiteP1(2,1:paths)=Random1(1:paths);
HermiteP1(3,1:paths)=Random1(1:paths).^2-1;
HermiteP1(4,1:paths)=Random1(1:paths).^3-3*Random1(1:paths);
HermiteP1(5,1:paths)=Random1(1:paths).^4-6*Random1(1:paths).^2+3;

YY(1:paths)=YY(1:paths) + ...
(YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+ ...
YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
(YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+ ...
YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+ ...
YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+ ...
YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
(YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+ ...
YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5) .*HermiteP1(2,1:paths) + ...
((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
(YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+ ...
YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2).*HermiteP1(3,1:paths) + ...
((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )).*HermiteP1(4,1:paths) + ...
(YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);

%Uncomment for fourth order monte carlo

%
%   YY(1:paths)=YY(1:paths) + ...
%       (YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
%       YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
%       (YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
%       YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
%       YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
%       (YCoeff0(1,1,4,1).*YY(1:paths).^Fp(1,1,4,1)+YCoeff0(1,2,3,1).*YY(1:paths).^Fp(1,2,3,1)+ ...
%       YCoeff0(2,1,3,1).*YY(1:paths).^Fp(2,1,3,1)+YCoeff0(1,3,2,1).*YY(1:paths).^Fp(1,3,2,1)+ ...
%       YCoeff0(2,2,2,1).*YY(1:paths).^Fp(2,2,2,1)+YCoeff0(3,1,2,1).*YY(1:paths).^Fp(3,1,2,1)+ ...
%       YCoeff0(1,4,1,1).*YY(1:paths).^Fp(1,4,1,1)+YCoeff0(2,3,1,1).*YY(1:paths).^Fp(2,3,1,1)+ ...
%       YCoeff0(3,2,1,1).*YY(1:paths).^Fp(3,2,1,1)+YCoeff0(4,1,1,1).*YY(1:paths).^Fp(4,1,1,1))*dtM^3 + ...
%        (YCoeff0(1,1,5,1).*YY(1:paths).^Fp(1,1,5,1)+YCoeff0(1,2,4,1).*yy(1:paths).^Fp(1,2,4,1)+ ...
%     YCoeff0(2,1,4,1).*YY(1:paths).^Fp(2,1,4,1)+YCoeff0(2,1,1,1).*yy(1:paths).^Fp(2,1,1,1)+ ...
%      YCoeff0(2,2,3,1).*YY(1:paths).^Fp(2,2,3,1)+ ...
%      YCoeff0(3,1,3,1).*YY(1:paths).^Fp(3,1,3,1)+YCoeff0(1,4,2,1).*YY(1:paths).^Fp(1,4,2,1)+ ...
%      YCoeff0(2,3,2,1).*YY(1:paths).^Fp(2,3,2,1)+YCoeff0(3,2,2,1).*YY(1:paths).^Fp(3,2,2,1)+ ...
%      YCoeff0(4,1,2,1).*YY(1:paths).^Fp(4,1,2,1)+YCoeff0(1,5,1,1).*YY(1:paths).^Fp(1,5,1,1)+ ...
%      YCoeff0(2,4,1,1).*YY(1:paths).^Fp(2,4,1,1)+ ...
%      YCoeff0(3,3,1,1).*YY(1:paths).^Fp(3,3,1,1)+YCoeff0(4,2,1,1).*YY(1:paths).^Fp(4,2,1,1)+ ...
%      YCoeff0(5,1,1,1).*YY(1:paths).^Fp(5,1,1,1))*dtM^4+ ...
%       ((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
%     (YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
%     YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5+ ...
%     (YCoeff0(1,1,3,2).*YY(1:paths).^Fp(1,1,3,2)+YCoeff0(1,2,2,2).*YY(1:paths).^Fp(1,2,2,2)+ ...
%     YCoeff0(2,1,2,2).*YY(1:paths).^Fp(2,1,2,2)+YCoeff0(1,3,1,2).*YY(1:paths).^Fp(1,3,1,2)+ ...
%     YCoeff0(2,2,1,2).*YY(1:paths).^Fp(2,2,1,2)+YCoeff0(3,1,1,2).*YY(1:paths).^Fp(3,1,1,2)).*dtM^2.5+ ...
%     (YCoeff0(1,1,4,2).*YY(1:paths).^Fp(1,1,4,2)+YCoeff0(1,2,3,2).*YY(1:paths).^Fp(1,2,3,2)+ ...
%     YCoeff0(2,1,3,2).*YY(1:paths).^Fp(2,1,3,2)+YCoeff0(1,3,2,2).*YY(1:paths).^Fp(1,3,2,2)+ ...
%     YCoeff0(2,2,2,2).*YY(1:paths).^Fp(2,2,2,2)+ YCoeff0(3,1,2,2).*YY(1:paths).^Fp(3,1,2,2)+ ...
%     YCoeff0(1,4,1,2).*YY(1:paths).^Fp(1,4,1,2)+YCoeff0(2,3,1,2).*YY(1:paths).^Fp(2,3,1,2)+ ...
%     YCoeff0(3,2,1,2).*YY(1:paths).^Fp(3,2,1,2)+YCoeff0(4,1,1,2).*YY(1:paths).^Fp(4,1,1,2)).*dtM^3.5) .*HermiteP1(2,1:paths) + ...
%     ((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
%     (YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
%     YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2+ ...
%     (YCoeff0(1,1,3,3).*YY(1:paths).^Fp(1,1,3,3)+YCoeff0(1,2,2,3).*YY(1:paths).^Fp(1,2,2,3)+ ...
%     YCoeff0(2,1,2,3).*YY(1:paths).^Fp(2,1,2,3) + YCoeff0(1,3,1,3).*YY(1:paths).^Fp(1,3,1,3)+ ...
%     YCoeff0(2,2,1,3).*YY(1:paths).^Fp(2,2,1,3)+YCoeff0(3,1,1,3).*YY(1:paths).^Fp(3,1,1,3)).*dtM^3).*HermiteP1(3,1:paths) + ...
%     ((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )+ ...
%     (YCoeff0(1,1,2,4).*YY(1:paths).^Fp(1,1,2,4)+YCoeff0(1,2,1,4).*YY(1:paths).^Fp(1,2,1,4)+ ...
%     YCoeff0(2,1,1,4).*YY(1:paths).^Fp(2,1,1,4))*dtM^2.5).*HermiteP1(4,1:paths) + ...
%     (YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);
%

YYMean(tt)=YYMean(tt)+sum(YY(1:paths))/paths;
end
YY(YY<0)=0;
disp('Original process average from monte carlo');
MCMean=sum(YY(:))/paths %origianl coordinates monte carlo average.
disp('Original process average from our simulation');
ItoHermiteMean=sum(y_w(wnStart+1:Nn-1).*ZProb(wnStart+1:Nn-1)) %Original process average from coordinates

disp('true Mean only applicble to standard SV mean reverting type models otherwise disregard');
TrueMean=theta+(x0-theta)*exp(-kappa*dt*Tt)%Mean reverting SDE original variable true average

MaxCutOff=30;
NoOfBins=round(200*gamma^2*5*sigma0);
[YDensity,IndexOutY,IndexMaxY] = MakeDensityFromSimulation_Infiniti_NEW(YY,paths,NoOfBins,MaxCutOff );
plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g');
%plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g',Z(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'b');

title(sprintf('Ito-Hermite Method VS Monte Carlo Density, x0 = %.2f,theta=%.2f,kappa=%.2f,gamma=%.3f,sigma=%.2f,T=%.2f', x0,theta,kappa,gamma,sigma0,T));%,sprintf('theta= %f', theta), sprintf('kappa = %f', kappa),sprintf('sigma = %f', sigma0),sprintf('T = %f', T));

%title('text',sprintf('x0 = %f', x0),sprintf('theta= %f', theta), sprintf('kappa = %f', kappa),sprintf('sigma = %f', sigma0),sprintf('T = %f', T))
%xlabel(a)
%title('Graphical Comparison of Ito-Hermite Method and Monte Carlo Method Density')
legend({'Ito-Hermite Density','Monte Carlo Density'},'Location','northeast')

str=input('red line is density of SDE from Ito-Hermite method, green is monte carlo.');

end

When you run the program with the following parametrs
ItoHermiteWilmottMeanCorrected08Exponential02(1.0,1.0,4,.65,1.5,4)
you should get output as
MCMean =0.999723371171040
ItoHermiteMean =0.999978998469931

Though the algorithm is quite good now but there is still need for improvement especially for smaller exponents towards gamma=.5 and I will try to come up with a better and more improved version of the program in a few days.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

First some background
We suppose we have a mean reverting stochastic volatility type equation(without loss of generality any equation with two or more terms in drift can be solved like this but I wanted a concrete example of the SDE that I am using)  with Ito-hermite method of moving and expanding/contracting grid. When we have the equation given as

$dx(t)=\kappa (\theta - x(t)) dt + \sigma x^{\gamma} dz(t)$

We used a transformed version of the above equation and simulated the dynamics of $w(t)=\frac{x(t)^{1-\gamma}}{(1-\gamma)}$. In original coordinates the dynamics of evolution of w(t) are given as

$dw(t)=\kappa \theta x^{-\gamma} dt - \kappa x^{1-\gamma} dt -.5 \gamma {\sigma}^2 x^{(\gamma-1)} dt + \sigma dz(t)$

In the equations that follow, I use the term $\mu$ for
$\mu=\int_0^{\Delta t} \big[\kappa \theta x^{-\gamma} dt - \kappa x^{1-\gamma} dt -.5 \gamma {\sigma}^2 x^{(\gamma-1)}\big] ds$
including its higher order expansion integrals. Please note that the term $\mu$ and also its higher derivatives like $\frac{\partial \mu}{\partial w}$ include the effect of multiplication of ${\Delta t}$ as it has been integrated.
For the friends who would like to know more and try doing related research on their own, here is what I added as correction that has improved the Ito-hermite density simulation algorithm. The correction term related to 2nd hermite has the form
$C_1 \Big [ \big[ \frac{\partial^2 w}{\partial Z^2} {\Delta t}-\frac{\partial \mu}{\partial w}\frac{\partial^2 w}{\partial Z^2} \big]-.5 \big[ (\frac{\partial w}{\partial Z})^2 {\Delta t}-\frac{\partial^2 \mu}{\partial w^2}(\frac{\partial w}{\partial Z})^2 \big] \Big] H_2(Z) f_2(Z)$
where
$f_2(Z)=\gamma (1+ (1-\gamma) {\sigma}^2) \exp(-.5(1-\gamma){\sigma}^2 Z^2)$
and $C_1=\frac{1}{{\pi}^2}$ is the constant used in the above equation and $H_2(Z)$ is the second hermite polynomial .
EDIT:[ The above equation is a small change from yesterday's program]

This greatly improves the density simulation algorithm for mean-reverting and other difficult SDEs as I showed in the graphs and program posted yesterday but there are still some places where density remains problematic especially at high volatilities, close to zero or when volatility exponent $\gamma$(gamma)  is close to .5 and sometimes also in the tail.
Therefore I have started to try higher hermite corrections and postulated a form as given below
$C_2 \Big [ \big[ \frac{\partial^3 w}{\partial Z^3} {\Delta t} -\frac{\partial \mu}{\partial w}\frac{\partial^3 w}{\partial Z^3} \big]-1/3 \big[ (\frac{\partial w}{\partial Z})^3 {\Delta t} -\frac{\partial^3 \mu}{\partial w^3}(\frac{\partial w}{\partial Z})^3 \big]$
$-2/3 \big[ (\frac{\partial w}{\partial Z})^2 \frac{\partial^2 w}{\partial Z^2} {\Delta t}-\frac{\partial^2 \mu}{\partial w^2}(\frac{\partial w}{\partial Z})^2 \frac{\partial^2 w}{\partial Z^2} \big] \Big] H_3(Z) f_3(Z)$
where
$f_3(Z)={\gamma}^2* (1+ (1-\gamma)^{1.5} {\sigma}^3) \exp(-.5(1-\gamma){\sigma}^2 Z^2)$
and $C_2$ is another constant and $H_3(Z)$ is the third hermite polynomial.
In initial experiments, it gives good results and further imrovements in the Ito-hermite algorithm when I just used the term
$-1/3 C_2 \big[ (\frac{\partial w}{\partial Z})^3 {\Delta t} -\frac{\partial^3 \mu}{\partial w^3}(\frac{\partial w}{\partial Z})^3 \big] H_3(Z) f_3(Z)$
since this would usually be the term with the largest effect.
But I will also including other terms in more detailed experiments and I also want to include the effect of fourth hermite polynomial calculated just like I calculated the effect of third hermite polynomial above.
It will take another two to three days to work with the program to find approximate coefficients and other changes in the program to appropriately add the effect of third and fourth order polynomial corrections.
In a few hours, I may post another matlab program with  several derivatives calculated so other friends could also play around with the program and make small changes to coefficients and other things to find the optimal algorithm. I am sure most friends would be at home and it would be a nice way to spend time for friends who are familiar with the program and would want to play with the algorithm on their own.

Last edited by Amin on March 31st, 2020, 4:58 pm, edited 1 time in total.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

In a few hours, I may post another matlab program with  several derivatives calculated so other friends could also play around with the program and make small changes to coefficients and other things to find the optimal algorithm. I am sure most friends would be at home and it would be a nice way to spend time for friends who are familiar with the program and would want to play with the algorithm on their own.

Here is the slightly updated version of the program. Important thing is that there is a new sub-function that calculates $\frac{d^4\mu}{dw^4}$ which was not available in the previous sub-function so that friends can play with correction terms of fourth hermite polynomials. $\frac{d^3 w}{dZ^3}$ and  $\frac{d^4w}{dZ^4}$  would generally not be available and might have to be calculated numerically but you could start with third and fourth hermite correction terms that only depend on $\frac{dw}{dZ}$ and  $\frac{d^2 w}{dZ^2}$ because they will have the major effect. Here is the new function. Please note that I have used in the second hermite correction term the following equation (slightly modified from yesterday's program)
$C_1 \Big [ \big[ \frac{\partial^2 w}{\partial Z^2} {\Delta t}-\frac{\partial \mu}{\partial w}\frac{\partial^2 w}{\partial Z^2} \big]-.5 \big[ (\frac{\partial w}{\partial Z})^2 {\Delta t}-\frac{\partial^2 \mu}{\partial w^2}(\frac{\partial w}{\partial Z})^2 \big] \Big] H_2(Z) f_2(Z)$
where
$f_2(Z)=\gamma (1+ (1-\gamma) {\sigma}^2) \exp(-.5(1-\gamma){\sigma}^2 Z^2)$
and $C_1=\frac{1}{{\pi}^2}$
while in the program I posted yesterday with graphs, I had used
$f_2(Z)= (1+{\gamma}^2 (1-\gamma) {\sigma}^2) \exp(-.5(1-\gamma){\sigma}^2 Z^2)$

This new change in $f_2(Z)$ further improves the Ito-hermite algorithm (I noticed that a lot of densities were still getting clipped at zero but that has further improved with this new correction in $f_2(Z)$)

function [TrueMean,ItoHermiteMean,MCMean] = ItoHermiteWilmottMeanCorrected08Exponential02Gamma(x0,theta,kappa,gamma,sigma0,T)

%Copyright Ahsan Amin. Infiniti derivatives Technologies.
%or skype ahsan.amin2999
%In this program, I am simulating the SDE given as
%dx(t)=kappa*(theta dt -x dt) +sigma x(t)^gamma dz(t)

%I have not directly simulated the SDE but simulated the transformed
%Besse1l process version of the SDE and then changed coordinates to retreive
%the SDE in original coo
%rdinates.
%The present program will analytically evolve only the Bessel Process version of the
%SDE in transformed coordinates.

dt=.125/16;   % Simulation time interval.%Fodiffusions close to zero
%decrease dt for accuracy.
%T=5;
Tt=128*T;%16*16;     % Number of simulation levels. Terminal time= Tt*dt; //.125/32*32*16=2 year;
OrderA=4;  %
OrderM=4;  %
%dtM=.125/8;%Monte carlo time interval size dtM.
%TtM=8*8;%Monte carlo number of simulation intervals.
dtM=dt*2;
TtM=Tt/2;

dNn=.2/1;   % Normal density subdivisions width. would change with number of subdivisions
Nn=48;  % No of normal density subdivisions
NnMidl=25;%One half density Subdivision left from mid of normal density(low)
NnMidh=26;%One half density subdivision right from the mid of normal density(high)
NnMid=4.0;

%theta=mu1/(-mu2);
%kappa=-mu2;

%x0=.25;   % starting value of SDE
beta1=0.0;
beta2=1.0;   % Second drift term power.
%gamma=.95;%50;   % volatility power.
%kappa=.5;   %mean reversion parameter.
%theta=.075;%mean reversion target

%you can specify any general mu1 and mu2 and beta1 and beta2.
%The present program is not perfect but quite reasonably good.
%if there is only one term in drift with beta=0, there will be minor
%errors. This case is yet not covered.

mu1=+1*theta*kappa;   %first drift coefficient.
mu2=-1*kappa;    % Second drift coefficient.
%sigma0=1.0;%Volatility value
alpha=1;% x^alpha is being expanded. This is currently for monte carlo only.
alpha1=1-gamma;

w(1:Nn)=x0^(1-gamma)/(1-gamma);

Z(1:Nn)=(((1:Nn)-5.5)*dNn-NnMid);

Z
str=input('Look at Z');
ZProb(1)=normcdf(.5*Z(1)+.5*Z(2),0,1)-normcdf(.5*Z(1)+.5*Z(2)-dNn,0,1);
ZProb(Nn)=normcdf(.5*Z(Nn)+.5*Z(Nn-1)+dNn,0,1)-normcdf(.5*Z(Nn)+.5*Z(Nn-1),0,1);

for nn=2:Nn-1
ZProb(nn)=normcdf(.5*Z(nn)+.5*Z(nn+1),0,1)-normcdf(.5*Z(nn)+.5*Z(nn-1),0,1);
%Above calculate probability mass in each probability subdivision.
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

sigma11(1:OrderA+1)=0;
mu11(1:OrderA+1)=0;
mu22(1:OrderA+1)=0;
sigma22(1:OrderA+1)=0;
% index 1 correponds to zero level since matlab indexing starts at one.
sigma11(1)=1;
mu11(1)=1;
mu22(1)=1;
sigma22(1)=1;

for k=1:(OrderA+1)
if sigma0~=0
sigma11(k)=sigma0^(k-1);
end
if mu1 ~= 0
mu11(k)=mu1^(k-1);
end
if mu2 ~= 0
mu22(k)=mu2^(k-1);
end
if sigma0~=0
sigma22(k)=sigma0^(2*(k-1));
end
end
%Ft(1:TtM+1,1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0; %General time powers on hermite polynomials
Fp(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers on coefficients of hermite polynomials.
Fp1(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;%General x powers for bessel transformed coordinates.

%YCoeff0 and YCoeff are coefficents for original coordinates monte carlo.
%YqCoeff0 and YqCoeff are bessel/lamperti version monte carlo.

YCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
YqCoeff0(1:(OrderA+1),1:(OrderA+1),1:(OrderA+1),1:(OrderA+1))=0;
%Pre-compute the time and power exponent values in small multi-dimensional arrays
YCoeff = ItoTaylorCoeffsNew(alpha,beta1,beta2,gamma); %expand y^alpha where alpha=1;
YqCoeff = ItoTaylorCoeffsNew(alpha1,beta1,beta2,gamma);%expand y^alpha1 where alpha1=(1-gamma)
YqCoeff=YqCoeff/(1-gamma); %Transformed coordinates coefficients have to be
%further divided by (1-gamma)
YqCoeffExp= ItoTaylorCoeffsNewO2Exp(alpha1,beta1,beta2,gamma);
YqCoeffExp=YqCoeffExp/(1-gamma);

Order2=2;
for k = 0 : (Order2)
for m = 0:k
l4 = k - m + 1;
for n = 0 : m
l3 = m - n + 1;
for j = 0:n
l2 = n - j + 1;
l1 = j + 1;
Fp1Exp(l1,l2,l3,l4) = (alpha1 + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));
YqCoeffExp0(l1,l2,l3,l4) =YqCoeffExp(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
end
end
end
end

for k = 0 : (OrderA)
for m = 0:k
l4 = k - m + 1;
for n = 0 : m
l3 = m - n + 1;
for j = 0:n
l2 = n - j + 1;
l1 = j + 1;
%Ft(l1,l2,l3,l4) = dtM^((l1-1) + (l2-1) + (l3-1) + .5* (l4-1));
Fp(l1,l2,l3,l4) = (alpha + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));
Fp1(l1,l2,l3,l4) = (alpha1 + (l1-1) * beta1 + (l2-1) * beta2 + (l3-1) * 2* gamma + (l4-1) * gamma ...
- (l1-1) - (l2-1) - 2* (l3-1) - (l4-1));

YCoeff0(l1,l2,l3,l4) =YCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);
YqCoeff0(l1,l2,l3,l4) =YqCoeff(l1,l2,l3,l4).*mu11(l1).*mu22(l2).*sigma22(l3).*sigma11(l4);

end
end
end
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
wnStart=1;%
d2wdZ2(1:Nn)=0;
dwdZ(1:Nn)=0;
yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
tic

for tt=1:Tt

kappa2=kappa*(1-gamma);
yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

%    [wMu0dt1,dwMu0dtdw1,d2wMu0dtdw21,d3wMu0dtdw31,c11,c21,c31,c41] = CalculateDriftAndVolOrder4(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt);

%[wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,c1,c2,c3,c4] = CalculateDriftAndVolOrderLowExponential(w,wnStart,Nn,YqCoeffExp0,Fp1Exp,gamma,dt,kappa2);

[wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,d4wMu0dtdw4,c1,c2,c3,c4] = CalculateDriftAndVolOrderLowExponentialNew(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt,kappa);

dw(wnStart:Nn)=c1(wnStart:Nn).*Z(wnStart:Nn) ;% ...
dw2(wnStart:Nn)=dw(wnStart:Nn).^2;
% d2w(wnStart:Nn)=c2(wnStart:Nn).*(Z(wnStart:Nn).^2-1);
% d3w(wnStart:Nn)=c3(wnStart:Nn).*(Z(wnStart:Nn).^3-3*Z(wnStart:Nn));
w(isnan(w)==1)=0;
wMu0dt(isnan(wMu0dt)==1)=0;
wMeanPrev=sum(ZProb(wnStart:Nn).*w(wnStart:Nn));%Calculate the mean.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

[w0,Sigma00,Sigma11,Sigma22,Sigma33,Sigma1,Sigma2,Sigma3,Sigma0] = CalculateHermiteRepresentationOfDensityOrderThreeNew(w,Z,wnStart,Nn,dNn,NnMidh,NnMidl);
d2wdZ2(wnStart:Nn)=d2wdZ2(wnStart:Nn)+ d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2+dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn);%+ ...

dwdZ(wnStart:Nn)=(Sigma00)+Sigma1(wnStart:Nn).*2.*Z(wnStart:Nn);%.*ExpDamp(wnStart:Nn);

H2(wnStart:Nn)=(Z(wnStart:Nn).^2-1);
H3(wnStart:Nn)=(Z(wnStart:Nn).^3-3*Z(wnStart:Nn));
H4(wnStart:Nn)=(Z(wnStart:Nn).^4-6*Z(wnStart:Nn).^2+3);
CTerm(wnStart:Nn)=(.5*d2wMu0dtdw2(wnStart:Nn).*dwdZ(wnStart:Nn).^2 - ...
1*dwMu0dtdw(wnStart:Nn).*d2wdZ2(wnStart:Nn));

Correction1(wnStart:Nn)=-gamma*(1+(1-gamma)*sigma0^2)*sqrt(3/4)/(22/7)^2.*CTerm(wnStart:Nn).*H2(wnStart:Nn);
Correction0(wnStart:Nn)=-gamma*(1+(1-gamma)*sigma0^2)*sqrt(3/4)/(22/7)^2.*(d2wdZ2(wnStart:Nn)-.5*dwdZ(wnStart:Nn).^2).*H2(wnStart:Nn);

if(tt==1)
w(wnStart:Nn)=wMeanPrev+exp(-kappa2*dt).*(wMu0dt(wnStart:Nn)+ ...
sign(exp(kappa2*dt).*(w(wnStart:Nn)-wMeanPrev)+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*exp(2*kappa2*dt).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn))));
end
if(tt>1)

w(wnStart:Nn)=wMeanPrev+exp(-kappa2*dt).*(wMu0dt(wnStart:Nn)+ ...
sign(exp(kappa2*dt).*(w(wnStart:Nn)-wMeanPrev)+dw(wnStart:Nn)).* ...
sqrt(abs(sign(w(wnStart:Nn)-wMeanPrev).*exp(2*kappa2*dt).*(w(wnStart:Nn)-wMeanPrev).^2+ ...
sign(dw(wnStart:Nn)).*dw2(wnStart:Nn))))+ ...
+dt *Correction0(wnStart:Nn).*exp(-.5*(1-gamma)*sigma0.^2.*Z(wnStart:Nn).^2) + ...
Correction1(wnStart:Nn).*exp(-.5*(1-gamma)*sigma0.^2.*Z(wnStart:Nn).^2);

end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Below are instability correction equations. There is instability in the
% SDE representation equation roughly between -.5 SD to +.5 SD in normal terms.
%Instability correction Algorithm takes two ends that are here given as
%lower end=w(NnMidl-2) and upper end=w(NnMidh+3).
%In the block below the interpolation starts from lower end.
%1st equation is
%And so on.
%There is a sixth equation that is not written below(since it takes us to
%the upper interpolation point which is already known) which is
%Sum of these six equations add up to the distance between upper and lower
%interpolation points.
%In the sum there are six instances of first derivative which is already
%calculated at the boundary, six instances of second derivative which is also
%already calculated at lower boundary and 18.0 instances of DeltaD1w which is
%unknown. This equation calculates the value of one unit of DeltaD1w addition
% I hope to find a better analytical method to tackle this issue. Friends
% are welcome to play with slightly changed related alternatives to the
%above interpolation.
CTheta=1.0;
D1wl=(11/6*w(NnMidl-2)-3*w(NnMidl-3)+3/2*w(NnMidl-4)-1/3*w(NnMidl-5))/dNn;
D2wl=(35/12*w(NnMidl-2) -26/3*w(NnMidl-3)+ 19/2*w(NnMidl-4)-14/3*w(NnMidl-5)+11/12*w(NnMidl-6))/dNn.^2;

%The last equation is not used since point w(NnMidh+3)is given but it is used in
%calculation of distance between two interpolation points. This distance is
%used to determine the value of DeltaD1w.

D1wh=(-11/6*w(NnMidh+3)+3*w(NnMidh+4)-3/2*w(NnMidh+5)+1/3*w(NnMidh+6))/dNn;
D2wh=(35/12*w(NnMidh+3) -26/3*w(NnMidh+4)+ 19/2*w(NnMidh+5)-14/3*w(NnMidh+6)+11/12*w(NnMidh+7))/dNn.^2;

%
w(NnMidh+2)=.5*whh(NnMidh+2)+.5*wll(NnMidh+2);
w(NnMidh+1)=.5*whh(NnMidh+1)+.5*wll(NnMidh+1);
w(NnMidh)=.5*whh(NnMidh)+.5*wll(NnMidh);
w(NnMidl)=.5*whh(NnMidl)+.5*wll(NnMidl);
w(NnMidl-1)=.5*whh(NnMidl-1)+.5*wll(NnMidl-1);

w1(1:Nn-1)=w(1:Nn-1);
w2(1:Nn-1)=w(2:Nn);

%         w1(1:Nn-2)=w(1:Nn-2);
%         w2(1:Nn-2)=w(2:Nn-1);
%         w3(1:Nn-2)=w(3:Nn);
%         w(w1(:)>w2(:))=w2(w1(:)>w2(:))-.25*(w3(w1(:)>w2(:))-w2(w1(:)>w2(:)));
w(w1(:)>w2(:))=0;%Be careful;might not universally hold;

w(w<0)=0.0;
for nn=1:Nn
if(w(nn)<=0)
wnStart=nn+1;
end
end

%%1st order mean correction. We know the mean in original
%%coordinates so I shift the density into original coordinates,
%%apply the mean correction and then transform again into Lamperti
%%coordinates. Algebra solves two equations in two unknowns.
%%Equations are Sum_0^N{(Y_w(wnStart:Nn)-Y0)*W0}=1 and the second
%%equation is Sum_0^N{Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0}=u0
%%Two unknows are Y0 and W0. u0 is known mean.
tt;
u0=theta+(x0-theta)*exp(-kappa*(tt*dt)); %analytic mean of the density

%If you are not using stochastic volatility, replace above with
%true mean otherwise results would become garbage.

Y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

Yn2Pn=sum(Y_w(wnStart:Nn).*Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
YnPn=sum(Y_w(wnStart:Nn).*ZProb(wnStart:Nn));
Pn=1.0;%%Sum(ZProb(1:Nn))
Y0=(Yn2Pn-u0*YnPn)/(YnPn-u0*Pn);

Y0Pn=Y0*Pn;
W0=1/(YnPn-Y0Pn);
YCorrected(wnStart:Nn)=Y_w(wnStart:Nn).*(Y_w(wnStart:Nn)-Y0)*W0;
wCorrected(wnStart:Nn)=(YCorrected(wnStart:Nn).^(1-gamma))./(1-gamma);
w(wnStart:Nn)=wCorrected(wnStart:Nn);

%I have disabled the mean correction. The above mean correction is only valid for
%standard mean reverting stochastic volatility type SDEs. To enable mean
%correction please uncomment the above last line in the block.

end

y_w(1:Nn)=0;
y_w(wnStart:Nn) = ((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));
Dfy_w(wnStart:Nn)=0;
Dfw(wnStart:Nn)=0;
for nn=wnStart+1:Nn-1
Dfy_w(nn) = (y_w(nn + 1) - y_w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
Dfw(nn) = (w(nn + 1) - w(nn - 1))/(Z(nn + 1) - Z(nn - 1));
%Change of variable derivative for densities
end
fy_w(1:Nn)=0;
fw(1:Nn)=0;
for nn = wnStart:Nn-1
fy_w(nn) = (normpdf(Z(nn),0, 1))/abs(Dfy_w(nn));%Origianl coordinates density
fw(nn) = (normpdf(Z(nn),0, 1))/abs(Dfw(nn));
end

toc
%str=input('Analytic Values calculated; Press a key to start monte carlo');

rng(29079137, 'twister')
paths=100000;
YY(1:paths)=x0;  %Original process monte carlo.
Random1(1:paths)=0;
YYMean(1:TtM)=0;
for tt=1:TtM
Random1=randn(size(Random1));
HermiteP1(1,1:paths)=1;
HermiteP1(2,1:paths)=Random1(1:paths);
HermiteP1(3,1:paths)=Random1(1:paths).^2-1;
HermiteP1(4,1:paths)=Random1(1:paths).^3-3*Random1(1:paths);
HermiteP1(5,1:paths)=Random1(1:paths).^4-6*Random1(1:paths).^2+3;

YY(1:paths)=YY(1:paths) + ...
(YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+ ...
YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
(YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+ ...
YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+ ...
YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+ ...
YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
(YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+ ...
YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5) .*HermiteP1(2,1:paths) + ...
((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
(YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+ ...
YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2).*HermiteP1(3,1:paths) + ...
((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )).*HermiteP1(4,1:paths) + ...
(YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);

%Uncomment for fourth order monte carlo

%
%   YY(1:paths)=YY(1:paths) + ...
%       (YCoeff0(1,1,2,1).*YY(1:paths).^Fp(1,1,2,1)+YCoeff0(1,2,1,1).*YY(1:paths).^Fp(1,2,1,1)+ ...
%       YCoeff0(2,1,1,1).*YY(1:paths).^Fp(2,1,1,1))*dtM + ...
%       (YCoeff0(1,1,3,1).*YY(1:paths).^Fp(1,1,3,1)+YCoeff0(1,2,2,1).*YY(1:paths).^Fp(1,2,2,1)+ ...
%       YCoeff0(2,1,2,1).*YY(1:paths).^Fp(2,1,2,1)+YCoeff0(1,3,1,1).*YY(1:paths).^Fp(1,3,1,1)+ ...
%       YCoeff0(2,2,1,1).*YY(1:paths).^Fp(2,2,1,1)+YCoeff0(3,1,1,1).*YY(1:paths).^Fp(3,1,1,1))*dtM^2 + ...
%       (YCoeff0(1,1,4,1).*YY(1:paths).^Fp(1,1,4,1)+YCoeff0(1,2,3,1).*YY(1:paths).^Fp(1,2,3,1)+ ...
%       YCoeff0(2,1,3,1).*YY(1:paths).^Fp(2,1,3,1)+YCoeff0(1,3,2,1).*YY(1:paths).^Fp(1,3,2,1)+ ...
%       YCoeff0(2,2,2,1).*YY(1:paths).^Fp(2,2,2,1)+YCoeff0(3,1,2,1).*YY(1:paths).^Fp(3,1,2,1)+ ...
%       YCoeff0(1,4,1,1).*YY(1:paths).^Fp(1,4,1,1)+YCoeff0(2,3,1,1).*YY(1:paths).^Fp(2,3,1,1)+ ...
%       YCoeff0(3,2,1,1).*YY(1:paths).^Fp(3,2,1,1)+YCoeff0(4,1,1,1).*YY(1:paths).^Fp(4,1,1,1))*dtM^3 + ...
%        (YCoeff0(1,1,5,1).*YY(1:paths).^Fp(1,1,5,1)+YCoeff0(1,2,4,1).*yy(1:paths).^Fp(1,2,4,1)+ ...
%     YCoeff0(2,1,4,1).*YY(1:paths).^Fp(2,1,4,1)+YCoeff0(2,1,1,1).*yy(1:paths).^Fp(2,1,1,1)+ ...
%      YCoeff0(2,2,3,1).*YY(1:paths).^Fp(2,2,3,1)+ ...
%      YCoeff0(3,1,3,1).*YY(1:paths).^Fp(3,1,3,1)+YCoeff0(1,4,2,1).*YY(1:paths).^Fp(1,4,2,1)+ ...
%      YCoeff0(2,3,2,1).*YY(1:paths).^Fp(2,3,2,1)+YCoeff0(3,2,2,1).*YY(1:paths).^Fp(3,2,2,1)+ ...
%      YCoeff0(4,1,2,1).*YY(1:paths).^Fp(4,1,2,1)+YCoeff0(1,5,1,1).*YY(1:paths).^Fp(1,5,1,1)+ ...
%      YCoeff0(2,4,1,1).*YY(1:paths).^Fp(2,4,1,1)+ ...
%      YCoeff0(3,3,1,1).*YY(1:paths).^Fp(3,3,1,1)+YCoeff0(4,2,1,1).*YY(1:paths).^Fp(4,2,1,1)+ ...
%      YCoeff0(5,1,1,1).*YY(1:paths).^Fp(5,1,1,1))*dtM^4+ ...
%       ((YCoeff0(1,1,1,2).*YY(1:paths).^Fp(1,1,1,2).*sqrt(dtM))+ ...
%     (YCoeff0(1,1,2,2).*YY(1:paths).^Fp(1,1,2,2)+YCoeff0(1,2,1,2).*YY(1:paths).^Fp(1,2,1,2)+ ...
%     YCoeff0(2,1,1,2).*YY(1:paths).^Fp(2,1,1,2)).*dtM^1.5+ ...
%     (YCoeff0(1,1,3,2).*YY(1:paths).^Fp(1,1,3,2)+YCoeff0(1,2,2,2).*YY(1:paths).^Fp(1,2,2,2)+ ...
%     YCoeff0(2,1,2,2).*YY(1:paths).^Fp(2,1,2,2)+YCoeff0(1,3,1,2).*YY(1:paths).^Fp(1,3,1,2)+ ...
%     YCoeff0(2,2,1,2).*YY(1:paths).^Fp(2,2,1,2)+YCoeff0(3,1,1,2).*YY(1:paths).^Fp(3,1,1,2)).*dtM^2.5+ ...
%     (YCoeff0(1,1,4,2).*YY(1:paths).^Fp(1,1,4,2)+YCoeff0(1,2,3,2).*YY(1:paths).^Fp(1,2,3,2)+ ...
%     YCoeff0(2,1,3,2).*YY(1:paths).^Fp(2,1,3,2)+YCoeff0(1,3,2,2).*YY(1:paths).^Fp(1,3,2,2)+ ...
%     YCoeff0(2,2,2,2).*YY(1:paths).^Fp(2,2,2,2)+ YCoeff0(3,1,2,2).*YY(1:paths).^Fp(3,1,2,2)+ ...
%     YCoeff0(1,4,1,2).*YY(1:paths).^Fp(1,4,1,2)+YCoeff0(2,3,1,2).*YY(1:paths).^Fp(2,3,1,2)+ ...
%     YCoeff0(3,2,1,2).*YY(1:paths).^Fp(3,2,1,2)+YCoeff0(4,1,1,2).*YY(1:paths).^Fp(4,1,1,2)).*dtM^3.5) .*HermiteP1(2,1:paths) + ...
%     ((YCoeff0(1,1,1,3).*YY(1:paths).^Fp(1,1,1,3) *dtM) + ...
%     (YCoeff0(1,1,2,3).*YY(1:paths).^Fp(1,1,2,3)+YCoeff0(1,2,1,3).*YY(1:paths).^Fp(1,2,1,3)+ ...
%     YCoeff0(2,1,1,3).*YY(1:paths).^Fp(2,1,1,3)).*dtM^2+ ...
%     (YCoeff0(1,1,3,3).*YY(1:paths).^Fp(1,1,3,3)+YCoeff0(1,2,2,3).*YY(1:paths).^Fp(1,2,2,3)+ ...
%     YCoeff0(2,1,2,3).*YY(1:paths).^Fp(2,1,2,3) + YCoeff0(1,3,1,3).*YY(1:paths).^Fp(1,3,1,3)+ ...
%     YCoeff0(2,2,1,3).*YY(1:paths).^Fp(2,2,1,3)+YCoeff0(3,1,1,3).*YY(1:paths).^Fp(3,1,1,3)).*dtM^3).*HermiteP1(3,1:paths) + ...
%     ((YCoeff0(1,1,1,4).*YY(1:paths).^Fp(1,1,1,4)*dtM^1.5 )+ ...
%     (YCoeff0(1,1,2,4).*YY(1:paths).^Fp(1,1,2,4)+YCoeff0(1,2,1,4).*YY(1:paths).^Fp(1,2,1,4)+ ...
%     YCoeff0(2,1,1,4).*YY(1:paths).^Fp(2,1,1,4))*dtM^2.5).*HermiteP1(4,1:paths) + ...
%     (YCoeff0(1,1,1,5).*YY(1:paths).^Fp(1,1,1,5)*dtM^2.0).*HermiteP1(5,1:paths);
%

YYMean(tt)=YYMean(tt)+sum(YY(1:paths))/paths;
end
YY(YY<0)=0;
disp('Original process average from monte carlo');
MCMean=sum(YY(:))/paths %origianl coordinates monte carlo average.
disp('Original process average from our simulation');
ItoHermiteMean=sum(y_w(wnStart+1:Nn-1).*ZProb(wnStart+1:Nn-1)) %Original process average from coordinates

disp('true Mean only applicble to standard SV mean reverting type models otherwise disregard');
TrueMean=theta+(x0-theta)*exp(-kappa*dt*Tt)%Mean reverting SDE original variable true average

MaxCutOff=30;
NoOfBins=round(200*gamma^2*8*sigma0);
[YDensity,IndexOutY,IndexMaxY] = MakeDensityFromSimulation_Infiniti_NEW(YY,paths,NoOfBins,MaxCutOff );
plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g');
%plot(y_w(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'r',IndexOutY(1:IndexMaxY),YDensity(1:IndexMaxY),'g',Z(wnStart+1:Nn-1),fy_w(wnStart+1:Nn-1),'b');

title(sprintf('Ito-Hermite Method VS Monte Carlo Density, x0 = %.2f,theta=%.2f,kappa=%.2f,gamma=%.3f,sigma=%.2f,T=%.2f', x0,theta,kappa,gamma,sigma0,T));%,sprintf('theta= %f', theta), sprintf('kappa = %f', kappa),sprintf('sigma = %f', sigma0),sprintf('T = %f', T));

%title('text',sprintf('x0 = %f', x0),sprintf('theta= %f', theta), sprintf('kappa = %f', kappa),sprintf('sigma = %f', sigma0),sprintf('T = %f', T))
%xlabel(a)
%title('Graphical Comparison of Ito-Hermite Method and Monte Carlo Method Density')
legend({'Ito-Hermite Density','Monte Carlo Density'},'Location','northeast')

str=input('red line is density of SDE from Ito-Hermite method, green is monte carlo.');

end


Here is the other new sub-function that you will need for the fourth derivative of drift $\mu$

function [wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,d4wMu0dtdw4,c1,c2,c3,c4] = CalculateDriftAndVolOrderLowExponentialNew(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt,kappa)

yy(wnStart:Nn)=((1-gamma)*w(wnStart:Nn)).^(1/(1-gamma));

Fp2=Fp1/(1-gamma);

dt1=(exp(kappa*dt)-1)/kappa;
dt2=(1 + exp(kappa* dt)* (-1 + kappa* dt))/kappa^2;

wMu0dt(wnStart:Nn)=(YqCoeff0(1,1,2,1).*yy(wnStart:Nn).^Fp1(1,1,2,1)+ ...
YqCoeff0(1,2,1,1).*yy(wnStart:Nn).^Fp1(1,2,1,1)+ ...
YqCoeff0(2,1,1,1).*yy(wnStart:Nn).^Fp1(2,1,1,1))*dt1 + ...
(YqCoeff0(1,1,3,1).*yy(wnStart:Nn).^Fp1(1,1,3,1)+ ...
YqCoeff0(1,2,2,1).*yy(wnStart:Nn).^Fp1(1,2,2,1)+ ...
YqCoeff0(2,1,2,1).*yy(wnStart:Nn).^Fp1(2,1,2,1)+ ...
YqCoeff0(1,3,1,1).*yy(wnStart:Nn).^Fp1(1,3,1,1)+ ...
YqCoeff0(2,2,1,1).*yy(wnStart:Nn).^Fp1(2,2,1,1)+ ...
YqCoeff0(3,1,1,1).*yy(wnStart:Nn).^Fp1(3,1,1,1))*dt2;
%     (YqCoeff0(1,1,4,1).*yy(wnStart:Nn).^Fp1(1,1,4,1)+ ...
%     YqCoeff0(1,2,3,1).*yy(wnStart:Nn).^Fp1(1,2,3,1)+ ...
%     YqCoeff0(2,1,3,1).*yy(wnStart:Nn).^Fp1(2,1,3,1)+ ...
%     YqCoeff0(1,3,2,1).*yy(wnStart:Nn).^Fp1(1,3,2,1)+ ...
%     YqCoeff0(2,2,2,1).*yy(wnStart:Nn).^Fp1(2,2,2,1)+ ...
%     YqCoeff0(3,1,2,1).*yy(wnStart:Nn).^Fp1(3,1,2,1)+ ...
%     YqCoeff0(1,4,1,1).*yy(wnStart:Nn).^Fp1(1,4,1,1)+ ...
%     YqCoeff0(2,3,1,1).*yy(wnStart:Nn).^Fp1(2,3,1,1)+ ...
%     YqCoeff0(3,2,1,1).*yy(wnStart:Nn).^Fp1(3,2,1,1)+ ...
%     YqCoeff0(4,1,1,1).*yy(wnStart:Nn).^Fp1(4,1,1,1))*dt^3;

dwMu0dtdw(wnStart:Nn)=(YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,1,1)))*dt1 + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,1,1,1)))*dt2;% + ...
%     (YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,1,4,1))+ ...
%     YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,2,3,1))+ ...
%     YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,1,3,1))+ ...
%     YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,3,2,1))+ ...
%     YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,2,2,1))+ ...
%     YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,1,2,1))+ ...
%     YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(1,4,1,1))+ ...
%     YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(2,3,1,1))+ ...
%     YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(3,2,1,1))+ ...
%     YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*((1-gamma).*w(wnStart:Nn)).^(-1+Fp2(4,1,1,1)))*dt^3;

d2wMu0dtdw2(wnStart:Nn)=(1-gamma).*((YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*(-1+Fp2(1,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*(-1+Fp2(1,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*(-1+Fp2(2,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,1,1)))*dt1 + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*(-1+Fp2(1,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*(-1+Fp2(1,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*(-1+Fp2(2,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*(-1+Fp2(1,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*(-1+Fp2(2,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*(-1+Fp2(3,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,1,1,1)))*dt2);% + ...
%     (YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*(-1+Fp2(1,1,4,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,1,4,1))+ ...
%     YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*(-1+Fp2(1,2,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,2,3,1))+ ...
%     YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*(-1+Fp2(2,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,1,3,1))+ ...
%     YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*(-1+Fp2(1,3,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,3,2,1))+ ...
%     YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*(-1+Fp2(2,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,2,2,1))+ ...
%     YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*(-1+Fp2(3,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,1,2,1))+ ...
%     YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*(-1+Fp2(1,4,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(1,4,1,1))+ ...
%     YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*(-1+Fp2(2,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(2,3,1,1))+ ...
%     YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*(-1+Fp2(3,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(3,2,1,1))+ ...
%     YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*(-1+Fp2(4,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-2+Fp2(4,1,1,1)))*dt^3);
%

d3wMu0dtdw3(wnStart:Nn)=(1-gamma)^2.*((YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*(-1+Fp2(1,1,2,1)).*(-2+Fp2(1,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*(-1+Fp2(1,2,1,1)).*(-2+Fp2(1,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*(-1+Fp2(2,1,1,1)).*(-2+Fp2(2,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,1,1)))*dt1 + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*(-1+Fp2(1,1,3,1)).*(-2+Fp2(1,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*(-1+Fp2(1,2,2,1)).*(-2+Fp2(1,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*(-1+Fp2(2,1,2,1)).*(-2+Fp2(2,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*(-1+Fp2(1,3,1,1)).*(-2+Fp2(1,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*(-1+Fp2(2,2,1,1)).*(-2+Fp2(2,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*(-1+Fp2(3,1,1,1)).*(-2+Fp2(3,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,1,1,1)))*dt2);% + ...
%     (YqCoeff0(1,1,4,1).*Fp1(1,1,4,1).*(-1+Fp2(1,1,4,1)).*(-2+Fp2(1,1,4,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,1,4,1))+ ...
%     YqCoeff0(1,2,3,1).*Fp1(1,2,3,1).*(-1+Fp2(1,2,3,1)).*(-2+Fp2(1,2,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,2,3,1))+ ...
%     YqCoeff0(2,1,3,1).*Fp1(2,1,3,1).*(-1+Fp2(2,1,3,1)).*(-2+Fp2(2,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,1,3,1))+ ...
%     YqCoeff0(1,3,2,1).*Fp1(1,3,2,1).*(-1+Fp2(1,3,2,1)).*(-2+Fp2(1,3,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,3,2,1))+ ...
%     YqCoeff0(2,2,2,1).*Fp1(2,2,2,1).*(-1+Fp2(2,2,2,1)).*(-2+Fp2(2,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,2,2,1))+ ...
%     YqCoeff0(3,1,2,1).*Fp1(3,1,2,1).*(-1+Fp2(3,1,2,1)).*(-2+Fp2(3,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,1,2,1))+ ...
%     YqCoeff0(1,4,1,1).*Fp1(1,4,1,1).*(-1+Fp2(1,4,1,1)).*(-2+Fp2(1,4,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(1,4,1,1))+ ...
%     YqCoeff0(2,3,1,1).*Fp1(2,3,1,1).*(-1+Fp2(2,3,1,1)).*(-2+Fp2(2,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(2,3,1,1))+ ...
%     YqCoeff0(3,2,1,1).*Fp1(3,2,1,1).*(-1+Fp2(3,2,1,1)).*(-2+Fp2(3,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(3,2,1,1))+ ...
%     YqCoeff0(4,1,1,1).*Fp1(4,1,1,1).*(-1+Fp2(4,1,1,1)).*(-2+Fp2(4,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-3+Fp2(4,1,1,1)))*dt^3);

d4wMu0dtdw4(wnStart:Nn)=(1-gamma)^3.*((YqCoeff0(1,1,2,1).*Fp1(1,1,2,1).*(-1+Fp2(1,1,2,1)).*(-2+Fp2(1,1,2,1)).*(-3+Fp2(1,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(1,1,2,1))+ ...
YqCoeff0(1,2,1,1).*Fp1(1,2,1,1).*(-1+Fp2(1,2,1,1)).*(-2+Fp2(1,2,1,1)).*(-3+Fp2(1,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(1,2,1,1))+ ...
YqCoeff0(2,1,1,1).*Fp1(2,1,1,1).*(-1+Fp2(2,1,1,1)).*(-2+Fp2(2,1,1,1)).*(-3+Fp2(2,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(2,1,1,1)))*dt1 + ...
(YqCoeff0(1,1,3,1).*Fp1(1,1,3,1).*(-1+Fp2(1,1,3,1)).*(-2+Fp2(1,1,3,1)).*(-3+Fp2(1,1,3,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(1,1,3,1))+ ...
YqCoeff0(1,2,2,1).*Fp1(1,2,2,1).*(-1+Fp2(1,2,2,1)).*(-2+Fp2(1,2,2,1)).*(-3+Fp2(1,2,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(1,2,2,1))+ ...
YqCoeff0(2,1,2,1).*Fp1(2,1,2,1).*(-1+Fp2(2,1,2,1)).*(-2+Fp2(2,1,2,1)).*(-3+Fp2(2,1,2,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(2,1,2,1))+ ...
YqCoeff0(1,3,1,1).*Fp1(1,3,1,1).*(-1+Fp2(1,3,1,1)).*(-2+Fp2(1,3,1,1)).*(-3+Fp2(1,3,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(1,3,1,1))+ ...
YqCoeff0(2,2,1,1).*Fp1(2,2,1,1).*(-1+Fp2(2,2,1,1)).*(-2+Fp2(2,2,1,1)).*(-3+Fp2(2,2,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(2,2,1,1))+ ...
YqCoeff0(3,1,1,1).*Fp1(3,1,1,1).*(-1+Fp2(3,1,1,1)).*(-2+Fp2(3,1,1,1)).*(-3+Fp2(3,1,1,1)).*((1-gamma).*w(wnStart:Nn)).^(-4+Fp2(3,1,1,1)))*dt2);% + ...

%b0=1;
%b1=A/2;
%b2=5/24*A^2;
%b3=1/16*A^3;
%b4=.01371*A^4;

%d1=1.0;
%d2=.75;
%d3=.40277;
%d4=.1718;
%d5=.06106;

%ExpIntegral=dt^1.5*(.4226+.4252*kappa*dt+.2554*kappa^2*dt^2+.1178*kappa^3*dt^3+.04312*kappa^4*dt^4);
ExpIntegral=dt^1.5*((1+kappa*dt/2+kappa^2*dt^2/6+kappa^3*dt^3/24+kappa^4*dt^4/120)- ...
sqrt(1/3+kappa*dt/4+7/60*kappa^2*dt^2+1/24*kappa^3*dt^3+31/2520*kappa^4*dt^4+1/360*kappa^5*dt^5));

c1(wnStart:Nn)=YqCoeff0(1,1,1,2).*yy(wnStart:Nn).^Fp1(1,1,1,2).*sqrt((exp(2*kappa*dt)-1)/(2*kappa))+ ...
(YqCoeff0(1,1,2,2).*yy(wnStart:Nn).^Fp1(1,1,2,2)+YqCoeff0(1,2,1,2).*yy(wnStart:Nn).^Fp1(1,2,1,2)+ ...
YqCoeff0(2,1,1,2).*yy(wnStart:Nn).^Fp1(2,1,1,2)).*ExpIntegral;%+ ...
%(YqCoeff0(1,1,3,2).*yy(wnStart:Nn).^Fp1(1,1,3,2)+YqCoeff0(1,2,2,2).*yy(wnStart:Nn).^Fp1(1,2,2,2)+ ...
%YqCoeff0(2,1,2,2).*yy(wnStart:Nn).^Fp1(2,1,2,2)+YqCoeff0(1,3,1,2).*yy(wnStart:Nn).^Fp1(1,3,1,2)+ ...
%YqCoeff0(2,2,1,2).*yy(wnStart:Nn).^Fp1(2,2,1,2)+YqCoeff0(3,1,1,2).*yy(wnStart:Nn).^Fp1(3,1,1,2)).*dt^2.5;

c2(wnStart:Nn)=0.0;%YqCoeff0(1,1,1,3).*yy(wnStart:Nn).^Fp1(1,1,1,3) *dt + ...
%  (YqCoeff0(1,1,2,3).*yy(wnStart:Nn).^Fp1(1,1,2,3)+YqCoeff0(1,2,1,3).*yy(wnStart:Nn).^Fp1(1,2,1,3)+ ...
%  YqCoeff0(2,1,1,3).*yy(wnStart:Nn).^Fp1(2,1,1,3)).*dt^2+ ...
%  (YqCoeff0(1,1,3,3).*yy(wnStart:Nn).^Fp1(1,1,3,3)+YqCoeff0(1,2,2,3).*yy(wnStart:Nn).^Fp1(1,2,2,3)+ ...
%  YqCoeff0(2,1,2,3).*yy(wnStart:Nn).^Fp1(2,1,2,3) + YqCoeff0(1,3,1,3).*yy(wnStart:Nn).^Fp1(1,3,1,3)+ ...
%  YqCoeff0(2,2,1,3).*yy(wnStart:Nn).^Fp1(2,2,1,3)+YqCoeff0(3,1,1,3).*yy(wnStart:Nn).^Fp1(3,1,1,3)).*dt^3;

c3(wnStart:Nn)=0.0;%YqCoeff0(1,1,1,4).*yy(wnStart:Nn).^Fp1(1,1,1,4)*dt^1.5 + ...
%   (YqCoeff0(1,1,2,4).*yy(wnStart:Nn).^Fp1(1,1,2,4)+YqCoeff0(1,2,1,4).*yy(wnStart:Nn).^Fp1(1,2,1,4)+ ...
%   YqCoeff0(2,1,1,4).*yy(wnStart:Nn).^Fp1(2,1,1,4))*dt^2.5;

c4(wnStart:Nn)=0.0;%YqCoeff0(1,1,1,5).*yy(wnStart:Nn).^Fp1(1,1,1,5)*dt^2.0;

end
Check: Run the program with the following parameters
ItoHermiteWilmottMeanCorrected08Exponential40(.25,.25,1,.65,.7,2)
ItoHermiteMean =0.249989792737747
MCMean =0.249235115749485

In the body of above main program, please change
Correction1(wnStart:Nn)=-gamma.*(1+(1-gamma)*sigma0^2)*sqrt(3/4)/(22/7)^2.*CTerm(wnStart:Nn).*H2(wnStart:Nn);
Correction0(wnStart:Nn)=-gamma.*(1+(1-gamma)*sigma0^2)*sqrt(3/4)/(22/7)^2.*(d2wdZ2(wnStart:Nn)-.5*dwdZ(wnStart:Nn).^2).*H2(wnStart:Nn);
To the following lines
Correction1(wnStart:Nn)=-gamma.*(1+(1-gamma)*sigma0^2)/(22/7)^2.*CTerm(wnStart:Nn).*H2(wnStart:Nn);
Correction0(wnStart:Nn)=-gamma.*(1+(1-gamma)*sigma0^2)/(22/7)^2.*(d2wdZ2(wnStart:Nn)-.5*dwdZ(wnStart:Nn).^2).*H2(wnStart:Nn);
i.e remove multiplication with *sqrt(3/4)
With this correction, you can verify that in the following correction term
$C_1 \Big [ \big[ \frac{\partial^2 w}{\partial Z^2} {\Delta t}-\frac{\partial \mu}{\partial w}\frac{\partial^2 w}{\partial Z^2} \big]-.5 \big[ (\frac{\partial w}{\partial Z})^2 {\Delta t}-\frac{\partial^2 \mu}{\partial w^2}(\frac{\partial w}{\partial Z})^2 \big] \Big] H_2(Z) f_2(Z)$
The value of $C_1$ is given as
$C_1=\frac{1}{{\pi}^2}$

and then check the following command
ItoHermiteWilmottMeanCorrected08Exponential40(.15,.15,1,.75,1,2)
to get
ItoHermiteMean =0.149989483848326
and
MCMean =0.149757391449352

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

Sorry friends for this mistake but in the last program I posted the following line has serious error.
The program has the line
[wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,d4wMu0dtdw4,c1,c2,c3,c4] = CalculateDriftAndVolOrderLowExponentialNew(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt,kappa);

but the actual line should be

[wMu0dt,dwMu0dtdw,d2wMu0dtdw2,d3wMu0dtdw3,d4wMu0dtdw4,c1,c2,c3,c4] = CalculateDriftAndVolOrderLowExponentialNew(w,wnStart,Nn,YqCoeff0,Fp1,gamma,dt,kappa2);

This is a function to calculate the drift and volatility and it receceives kappa2 as argument which is different from kappa. The value of
kappa2= kappa*(1-gamma) and this is kappa for exponential that is used to cancel one term in the solution of SDE (and different from the kappa in the original SDE equation). unfortunately in the very last program, I failed to change kappa value to kappa2 in the input arguments and it uses kappa of original SDE equation which is very wrong. This was not a problem in the program I posted yesterday and this mistake is only in the program I posted today. I would request friends to fix this line in the program. And sorry again for any time that was wasted due to this error.

And we have to roll back to previous days value of correction terms since the new values were checked with the wrong program that took the value of kappa where I needed to use the value of kappa2.
Now the right value of correction terms would be
$C_1 \Big [ \big[ \frac{\partial^2 w}{\partial Z^2} {\Delta t}-\frac{\partial \mu}{\partial w}\frac{\partial^2 w}{\partial Z^2} \big]-.5 \big[ (\frac{\partial w}{\partial Z})^2 {\Delta t}-\frac{\partial^2 \mu}{\partial w^2}(\frac{\partial w}{\partial Z})^2 \big] \Big] H_2(Z) f_2(Z)$
where
$f_2(Z)= (1+{\gamma}^2 (1-\gamma) {\sigma}^2) \exp(-.5(1-\gamma){\sigma}^2 Z^2)$
and $C_1=\frac{\sqrt{3/4}}{{\pi}^2}$
These are the values I used in the program that I posted yesterday night.
If there is any change in the parameter values on my side, I will let friends know in another post.

Amin
Topic Author
Posts: 2318
Joined: July 14th, 2002, 3:00 am

### Re: Breakthrough in the theory of stochastic differential equations and their simulation

Dear friends, I was trying to think how to derive the proper equations for the evolution of density in Ito-hermite method. I was able to find some other interesting equations that I decided to share here with friends.
Before I write the main equations, I want to mention that this idea is based on equivalence between two densities. I take one density that is standard normal called Z (could also be taken as brownian motion) and then I try to find the evolution of a particular point on the density of SDE variable X(t,Z) and this particular point is related to a specific fixed value of Z which is already known. Z is a static density that does not change with time at all.
I will write a few equations first that we will use later in the main derivation. Here are the equations

$P(Z)=P(X,t) |{\frac{\partial X}{\partial Z}}|$   Follows from change of variable formula for densities.
$\frac{\partial P(X,t)}{\partial X}= \frac{\partial p(Z)}{\partial Z} (\frac{\partial Z}{\partial X})^2$

$\frac{d}{dt} \big[ {\frac{\partial X}{\partial Z}} \big]=\frac{\partial}{\partial X}\big[ {\frac{\partial X}{\partial Z}} \big] \frac{\partial X}{\partial t}$
$=-{(\frac{\partial X}{\partial Z})}^2 \frac{\partial^2 Z}{\partial X^2} \frac{\partial X}{\partial t}$

$\frac{\partial P(X,t)}{\partial t} = f(X)$ where f(X) is the fokker planck equation.

Now we come towards the main equation derivation(which might as well possibly be wrong)

Since standard normal is a static density and it does not change with time, we can write

$\frac{d}{dt}[p(Z)]=0$
since we can write densities in terms of each other using change of variable formula, we can write
$\frac{d}{dt}[p(X,t)\frac{\partial X}{\partial Z}]=0$
taking the derivatives, we can expand the above equation as
$\big [ \frac{\partial p(X,t)}{\partial t}+\frac{\partial p(X,t)}{\partial X}\frac{\partial X}{\partial t} ]\frac{\partial X}{\partial Z}$
$-p(X,t){(\frac{\partial X}{\partial Z})}^2 \frac{\partial^2 Z}{\partial X^2} \frac{\partial X}{\partial t}=0$

using
$P(Z)=P(X,t) |{\frac{\partial X}{\partial Z}}|$
$\frac{\partial P(X,t)}{\partial X}= \frac{\partial p(Z)}{\partial Z} (\frac{\partial Z}{\partial X})^2$
and $\frac{\partial P(X,t)}{\partial t} = f(X)$
in the above equation ,we get
$\frac{\partial X(Z,t)}{\partial t} =\frac{ \frac{\partial p(X,t)}{\partial t}}{ \big[ (p(Z) \frac{\partial^2 Z}{\partial X^2}+ Z p(Z) ({\frac{\partial Z}{\partial X}})^2\big]}$

using the above equation, we can probably solve every SDE so that we can solve for evolution of X as a function of Z and t. More detailed post and notes tomorrow.

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