- Cuchulainn
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QuoteOriginally posted by: list1Actually for about a month I look at SDEs from math point of view. I found a simple proof of the well known result and start to think about its generalization. I bumped with a technical difficulty which might be known or not as I am not sure about. It relates may be to closed form solution. It is well known that solution of the linear sde admits closed form representation. Whether does this result is known for two dimensional case? Actually I need the result that states positiveness of the solution if the initial value is positive. I need it for pure mathematics. On the other hand it can be used in finance too as a model in which correlated indexes or currencies are modeled by multidimensional system of SDEs. I will appreciate to a suggested hint.I find the term 'pure mathematics' a bit artificial. Quote states positiveness of the solution if the initial value is positiveThis is a qualitative property of the solution that you should be able to prove without actually having a closed form. It is called the maximum principle in PDE.//BTW the bespoke calculus of variations example and 'solved' by the Euler equation was discovered e as follows:QuoteIn solving optimisation problems in function spaces, Euler made extensive use of this `methodof finite differences'. By replacing smooth curves by polygonal lines, he reduced the problem offinding extrema of a function to the problem of finding extrema of a function of n variables, andthen he obtained exact solutions by passing to the limit as n ! 1. In this sense, functions canbe regarded as `functions of infinitely many variables' (that is, the infinitely many values of x(t)at different points), and the calculus of variations can be regarded as the corresponding analog ofdifferential calculus of functions of n real variables.

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1. I find the term 'pure mathematics' a bit artificial. // Pure math is when one is proving a theorem and do not think about its applications and focus only to express proof shortly and in most comprehensive form.2. This is a qualitative property of the solution that you should be able to prove without actually having a closed form. It is called the maximum principle in PD// It will be good. Also I have not worked much with matrices and actually could not understand quite simple questions. For example if A is a 2[$]\times[$]2 matrix. Is it possible to present explicitly elements of of the matrix [$]e^A[$]? It might be written somewhere and not a difficult question.3. Its often happen in math that similar problem is simple and well resolved while what one is needed is a problem having not obvious solution.

Last edited by list1 on February 8th, 2016, 11:00 pm, edited 1 time in total.

- Cuchulainn
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QuoteOriginally posted by: list1For example if A is a 2[$]\times[$]2 matrix. Is it possible to present explicitly elements of of the matrix [$]e^A[$]? It might be written somewhere and not a difficult question.For exp(Matrix) this might be a good startAlso, textbooks on ODEs will usually have a chapter or two on this subject. Best to start with simple examples. It's a real deep theory. It underlies much of PDE/FDM. Depending on the structure of A, it might be possible to find an exact representation for exp(A) etc. Maybe the matrix is diagonalizable, then it becomes easy.

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- Cuchulainn
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You're welcome!

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QuoteIn 1918 Einstein published the paper ÜBER GRAVITATIONSWELLEN [1] in which, for the first time, the effect ofgravitational waves was calculated, resulting in his famous quadrupole formula (QF). Einstein was forced to thispublication due to a serious error in his 1916 paper [2], where he had developed the linear approximation (weakfield)scheme to solve the field equations of general relativity (GR). In analogy to electrodynamics, whereaccelerated charges emit electromagnetic waves, the linearized theory creates gravitational waves, popagatingwith the speed of light in the (background) Minkowski space-time. A major difference: Instead of a dipolemoment, now a quadrupole moment is needed. Thus sources of gravitational waves are objects like a rotatingdumbbell, e. g. realized by a binary star system.

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- Cuchulainn
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There is a third way between searching for an explicit solution (a big favourite!) and a numerical solution

In mathematics, the**qualitative theory of differential equations** studies the behavior of differential equations by means other than finding their solutions. It originated from the works of Henri Poincaré and Aleksandr Lyapunov. There are relatively few differential equations that can be solved explicitly, but using tools from analysis and topology, one can "solve" them in the qualitative sense, obtaining information about their properties.^{[1]}

In mathematics, the

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