using fftsg.c from General Purpose FFT (Fast Fourier/Cosine/Sine Transform) Package,

http://momonga.t.u-tokyo.ac.jp/~ooura/fft.html

Copyright Takuya OOURA, 1996-2001 (Email: ooura@kurims.kyoto-u.ac.jp or ooura@mmm.t.u-tokyo.ac.jp) 


