Consider the random walk with drift model xt = d + + wt, for t = 1, 2, . . , with x0 = 0, where wt is white noise with variance s2w . (a) show that the model can be written as xt = dt + åt k=1 wk. (b) find the mean function and the autocovariance function of xt. (c) argue that xt is not stationary. (d) show rx(t 1, t) = q t ! 1 as t ! ¥. what is the implication of this result? (e) suggest a transformation to make the series stationary, and prove that the transformed series is stationary. 2.6. would you treat the global temperature