Hi Fatemeh, > I need a subroutine in Fortran90 for "Fast Fourier Transform of a > step-function in 3-dimonsional" . Would you please help me? For Fortran, I recommend the "Numerical Recipes in Fortran" books: http://www.amazon.com/dp/052143064X/ http://www.amazon.com/dp/0521437210/ http://www.amazon.com/dp/0521574390/ Depending on the problem you are working on, it may be easier to use Mathematica rather than Fortran. HTH, --Eljay
