4.10 Consider the QAR model for a stationary Gaussian random field in Example 4.6. If c = −ab, show that the spectral density takes the form f(????) = ????2

|1 − aei????[1] − bei????[2] + abei(????[1]+????[2])|

−2

= ????2

|1 − aei????[1]

|

−2

|1 − bei????[2]

|

−2

.

Hence, deduce that this model is separable in ????[1] and ????[2] (see Exercise 2.8).