Suppose that {Yt } is stationary with autocovariance function γk. (a) Show that Wt = ∇Yt = Yt − Yt − 1 is stationary by finding the mean and autocovariance function for {Wt }. (b) Show that Ut = ∇2Yt = ∇[Yt − Yt−1] = Yt − 2Yt−1 + Yt−2 is stationary. (You need not find the mean and autocovariance function for {Ut }.)