Question for the HP guys....

[parisse]

The GSL is not active when giac is compiled for the Prime.
There are two main FFT implementations, one for complex<double> data, and one for modular integers. The first one has prototype

Code:

void fft(std::complex<double> * f,int n,const std::complex<double> * w,int m,complex< double> * t)

it first checks if the size is even, if it is odd it tries to find a factor, if not it applies slow FFT. For even size, there is no special code if n is divisible by 4. The implementation for complex<double> is not fully optimized (the integer modular fft is more optimized).