FFTW is a C subroutine library for computing the discrete Fourier transform (DFT) in one or more dimensions, of arbitrary input size, and of both real and complex data (as well as of even/odd data, i.e. the discrete cosine/sine transforms or DCT/DST). The FFTW package was developed at MIT by Matteo Frigo and Steven G. Johnson.

licenseGNU General Public Licence
helpmanual(html), (pdf), using fftw