use Math::FFT;
  my $PI = 3.1415926539;
  my $N = 64;
  my ($series, $other_series);
  for (my $k=0; $k<$N; $k++) {
      $series->[$k] = sin(4*$k*$PI/$N) + cos(6*$k*$PI/$N);
  }
  my $fft = new Math::FFT($series);
  my $coeff = $fft->rdft();
  my $spectrum = $fft->spctrm;
  my $original_data = $fft->invrdft($coeff);

  for (my $k=0; $k<$N; $k++) {
      $other_series->[$k] = sin(16*$k*$PI/$N) + cos(8*$k*$PI/$N);
  }
  my $other_fft = $fft->clone($other_series);
  my $other_coeff = $other_fft->rdft();
  my $correlation = $fft->correl($other_fft);