4. Assume the following unit root model without a drift Yt Et ~ Yt-1 + Et, Yo = 0, iid (0,0), (UR Model) and this is sometimes called as the "stochastic trend model". (a) Using a recursive substitution, show that Yt==1&j. (b) Derive the expression for Var (Yt), and confirm that it is increasing as t gets larger. Is Y covariance stationary? (c) Compute the impulse response function IRF, ay with the integer j = (1, t - 1). det-j