Suppose Yt = Xt + et, where {et} are normal white noise with mean 0 and variance σ^2. The {Xt} process is a stationary AR(1) defined by Xt = φXt−1 + Zt, where {Zt} is a zero mean normal white noise process with variance σ^2. Assume that Zt is independent of Xt−1, Xt−2, ... Assume additionally that E(etZs) = 0 for all t and s.
(a) Show that {Yt} is stationary and find its auto-covariance function, γk.
(b) Show that the process {Ut}, where Ut = Yt − φYt−1 = (1 − φB)Yt, has nonzero correlation only at lag 1.