Hi,
I am quite new to gsl and have a beginner's question.
I computed the FFT coefficients of a double array using
gsl_fft_real_transform. On the result I applied
gsl_fft_halfcomplex_unpack in order to get the familiar representation
as series of complex numbers.
From that series I'd like to evaluate the function that is represented
by these numbers which can be considered as approximation of the
original data.
I read the manual and used a piece of code like this
double to_real (double * c, unsigned int k, double t)
{
double cr = c[2*k];
double ci = c[2*k+1];
double a = 2.0 * M_PI * k/N * t;
// c * exp (2 * Pi * i * k/N * t)
return cr * cos(a) - ci * sin(a);
}
// return f(t)
// f is series of complex numers computed by
// gsl_fft_real_transform + gsl_fft_halfcomplex_unpack
double eval_trafo (double * f, double t)
{
double val = 0.0;
for (unsigned int k=0; k < N; k++)
{
val += to_real (f, k, t);
}
val /= N;
return val;
}
But obviously, that does not represent the original function. What am I
missing?
Would be great if anyone could give ne a hint.
Georg-Johann
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