Hello all, I have short question at the end , but i wil gve short background. The subject is regarding the split step fourier method (SSFM) adn i will be gratefull if someone who know the method can help. i have the set of 2 equations: Utt=Uzz-a*U+i*P-(P^2)*U Pt=-i*P-(P^2)*U where: P, U are the function need to be find P(z,t) U(z,t) i = sqrt(-1) a=constant I try to do the split step fourier method (SSFM) on this set, the problem is that the first equation derivative are from order 2, and in all the examples i saw it was from order 1. I try to seperate to linear part and nonlinear part: for the first equation: U_linear_tt=Uzz-a*U U_nonlinear_tt= i*P-(P^2)*U and for the second equation: P_linear_t=-i*P P_nonlinear_t=-(P^2)*U now when i try to do the fourier transform at the linear part, for the U function, it is problem because the derivative is second order. if it was from first order the fourier transform for the linear part becomes: Fourier_U_linear(t+dt,z)=exp{(-w^2-a)*dt} how do i modified it on my case when the derivative is second order ? thanks