Fourier differentiation

Fourier spectral differentiation of a periodic real function

W = FOURIERDIFF(V,M,INTERVAL,N) returns the M-th derivative of function
V sampled at N (even) equidistant points on INTERVAL.
W = FOURIERDIFF(V,M,INTERVAL) returns the M-th derivative of function V
sampled at 128 equidistant points on INTERVAL.
W = FOURIERDIFF(V,M) returns the M-th derivative of function V sampled
at 128 equidistant points on the default interval [0 2*pi].
W = FOURIERDIFF(V) returns the first derivative of function V sampled
at 128 equidistant points on the default interval [0 2*pi].
M must be a non-negative integer and N must be even.
Note that the approximation is poor if V is not periodic.