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# numpy.fft.fftfreq

`numpy.fft.``fftfreq`(n, d=1.0)

  Return the Discrete Fourier Transform sample frequencies.

  The returned float array contains the frequency bins in cycles/unit (with zero at the start) given a window length n and a sample spacing d:

      f = [0, 1, ..., n/2-1, -n/2, ..., -1] / (d*n)         if n is even
      f = [0, 1, ..., (n-1)/2, -(n-1)/2, ..., -1] / (d*n)   if n is odd

  Parameters : | n : int Window length. d : scalar Sample spacing.
  Returns : | out : ndarray The array of length n, containing the sample frequencies.

  Examples

      >>> signal = np.array([-2, 8, 6, 4, 1, 0, 3, 5], dtype=float)
      >>> fourier = np.fft.fft(signal)
      >>> n = signal.size
      >>> timestep = 0.1
      >>> freq = np.fft.fftfreq(n, d=timestep)
      >>> freq
      array([ 0.  ,  1.25,  2.5 ,  3.75, -5.  , -3.75, -2.5 , -1.25])

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