fftw: 
fftw: FFTW is a free collection of fast C routines for computing the
fftw: Discrete Fourier Transform in one or more dimensions. It includes
fftw: complex, real, symmetric, and parallel transforms, and can handle
fftw: arbitrary array sizes efficiently. FFTW is typically faster than
fftw: other publicly-available FFT implementations, and is even
fftw: competitive with vendor-tuned libraries.
fftw:
fftw: Homepage: http://www.fftw.org/
fftw: