I have to solve a differential equation in MATLAB in (x,y) involving partial derivatives. My solution is a function of (x,y). I want to use fft differentiation( spectral method) to solve my partial derivatives.But i am stuck. I am using the following algorithm % fderiv = ifft((i*k).^p .* fft(f)); % pth derivative of f(matrix containing functional values). But when i try this for size(f)=2 ..it is not showing me the correct result. my x is symmetric (-L,L) 1. does this method work with periodic f only or non periodic functions can be differentiated as well? If yes..how? 2. How to choose k in above program?