start).  For instance, if
the sample spacing is in seconds, then the frequency unit is cycles/second.

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, optional
    Sample spacing (inverse of the sampling rate). Defaults to 1.
device : str, optional
    The device on which to place the created array. Default: ``None``.
    For Array-API interoperability only, so must be ``"cpu"`` if passed.

    .. versionadded:: 2.0.0

Returns
-------
f : ndarray
    Array of length `n` containing the sample frequencies.

Examples
--------
>>> import numpy as np
>>> 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, -2.5 , -1.25])

ú