Topic: Model structures Write down the model equation for a stochastic process (Yt) generated by each of the following time series models, in each case expressing Yt in terms of previously-occurring quantities. State any restrictions on the model parameters that will ensure, if possible, that (Yt) is (a) stationary (b) invertible. i MA(2) with zero mean. ii ARMA(2,1) with non-zero mean . iii SARIMA(1, 0, 0) (0, 1, 2)4 without a constant.

Topic: Spectral Density Graph the theoretical spectral density for an MA(1) process with 1 = 0.6. Interpret the implications of the shape of the spectrum on the possible plots of the time series values.