Assignment:
Are the AR(2) models
Xt = 1.2Xt-1 - .85Xt-2+ wt, Yt = 1.8Yt-1 - 1.06Yt-2 + wt causal? For those that are compute the ACF. Do this first using your favorite mathematical method, then plot your answer out to lag 10 and compare it with what is produced by ARMAacf () to check your answer.