[Help-gsl] Quick FFT question

[Nghia Ho]

Hi,

I need to implement an algorithm that makes use of the FFT to do some basic 
signal processing. Basically, given an array of numbers I would like to extract 
the coefficients of the N lowest frequency components. 

The GSL document says the results are laid out such that index from 0 to N/2 
are the positive frequencies and the rest are negative. My mathematical 
knowledge of FFT is limited so please bare with me on the next question. I see 
the real values are the same for both positive/negative side, but imaginary 
values flip. So if I take the magnitude (|Re + Imz|) of the first N values of 
the positive side would that give me what I want?

Thanks,
Nghia