You are in the HTML documentation section of the FFTEASY web site. This documentation is also available as a postscript file. You can also go to the main FFTEASY page or just download ffteasy.c.
The documentation for FFTEASY is divided into four sections for people asking four different kinds of questions:
What is this? This section is very short and everyone even thinking about using FFTEASY should read it. It contains a two paragraph description of what FFTEASY is and isn't good for and the obligatory paragraph about what you are and aren't allowed to do with FFTEASY. (Don't worry, it's completely free.)
How do I use this? The section is the heart of the FFTEASY documentation. If you don't care about the inner workings of the routines but simply want to stick them in your code and start doing Fourier transforms with them then you can read this section and go no further. This section includes a list of the C functions included in FFTEASY and how to call them from your C program. It also describes the conventions used by FFTEASY for defining Fourier transforms and for storing their output in C arrays.
More details. That wasn't satisfying. Can you tell me more about what FFTEASY is doing? If this is you, then read this section for a description of how FFTEASY does its calculations. This makes for fun summer reading if you're a serious nerd. On a more practical level this section may be helpful if you want to modify the code in some way.
Can you give me some examples? This section lists sample codes for calling the FFTEASY routines, including code for defining an array of complex numbers in frequency space with the correct symmetries for transforming to a real function using FFTEASY.