ARMA models combine the AR and MA techniques to generate a forecast. I recommend reading this post to learn more about AR, MA, and ARMA models.

Subset, Seasonal, and Factored ARMA Models . The simplest way to specify an ARMA model is to give the order of the AR and MA parts with the P= and Q= options. When you do this, the model has ...

Hence, when i=p, you can substitute for in the right-hand side of equation (2) and close the system of equations.. This substitution results in the following model in the state space form z t+1 = Fz t ...