Recall from Problem 4.32 that an LTI system cannot produce frequencies at its output that are different from those applied in its input. Thus if a system creates “new” frequencies, it must be nonlinear and/or time varying and determine the output spectra when the input spectrum is 

(a) y (n) = x(2n)

(b) y (n) = x²(n)

(c) y (n) = (cos πn) x (n) 

|wl < n/4 1. X (w) 0.