Newbie here, please. I took an online course to learn European call option pricing using Fourier Transform, and attach my notes here to provide context and ask a question pertaining to the notes. Please refer to page 5 specifically where the Fubini rule is applied to re-arrange the inner and the outer integrals, and ultimately show that the formula for the modified call can be represented as some multiple of the characteristic function of the logarithm of the stock price.
One of the things that isn't explained on the lecture is how the inner integral changes from being k through infinity to negative infinity through s. And that's all still for the call option pricing. I would like to understand how this works for put option pricing. What would the inner integral change to? Ultimately, the goal is to represent the function for the modified call as some multiple of the characteristic function.
I have tried 3 different approaches to analytically address the inner integral, and have attached a PDF showing those 3 alternative approaches on 3 pages. The first one sets the inner integral from negative infinity through s, the second sets it from 0 through s, and the final one sets it from negative infinity to zero. However, one of those cases arrives at a form that looks like a multiple of the characteristic function.
So, I'm back to the drawing board and wondering how the integrals on Page 5 need to be modified and what limits they should pertain to, in order to price a put option instead of a call option.
Most grateful for anyone's guidance.
Kind Regards,
SK.
