FFTPACK is a package of Fortran subroutines for the fast Fourier transform.
It includes complex, real, sine, cosine, and quarter-wave transforms.
It was developed by Paul Swarztrauber of the National Center for Atmospheric Research, and is included in the general-purpose mathematical library SLATEC.
Much of the package is also available in C and Java translations.
The pm_fftpack module of the ParaMonte library offers a 100% type-kind-generic multi-precision implementation of FFTPACK in modern Fortran.