"Fast Fourier Transform of a step-function in 3-dimonsional"

[John Love-Jensen]

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