diff(x, order=1, period=2*pi) -> y
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.
For odd order and even len(x), the Nyquist mode is taken zero.