
    Return k-th derivative (or integral) of a periodic sequence x.

    If x_j and y_j are Fourier coefficients of periodic functions x
    and y, respectively, then::

      y_j = pow(sqrt(-1)*j*2*pi/period, order) * x_j
      y_0 = 0 if order is not 0.

    Parameters
    ----------
    x : array_like
        Input array.
    order : int, optional
        The order of differentiation. Default order is 1. If order is
        negative, then integration is carried out under the assumption
        that ``x_0 == 0``.
    period : float, optional
        The assumed period of the sequence. Default is ``2*pi``.

    Notes
    -----
    If ``sum(x, axis=0) = 0`` then ``diff(diff(x, k), -k) == x`` (within
    numerical accuracy).

    For odd order and even ``len(x)``, the Nyquist mode is taken zero.

    