FFTW, the Fastest Fourier Transform in the West.  FFTW is a comprehensive
  collection of fast C routines for computing the discrete Fourier transform
  (DFT) in one or more dimensions, of both real and complex data, and of
  arbitrary input size.  FFTW also includes parallel transforms, documented
  elsewhere.  We assume herein that the reader is already familiar with the
  properties and uses of the DFT that are relevant to her application.
  Otherwise, see e.g. `The Fast Fourier Transform' by E. O. Brigham
  (Prentice-Hall, Englewood Cliffs, NJ, 1974).  Our web page also has links to
  FFT-related information online, see

	http://theory.lcs.mit.edu/~fftw