For AR(P) process, I once read the following property:

P corresponds to the projection on the space $\bar{sp}\{x_2,...x_k\}$, which is defined as follows

I do not know how does equation (3.4.1) was derived
For AR(P) process, I once read the following property:

P corresponds to the projection on the space $\bar{sp}\{x_2,...x_k\}$, which is defined as follows

I do not know how does equation (3.4.1) was derived
Copyright © 2021 JogjaFile Inc.
The RHS of (3.4.1) belongs to the span, $X_t$ equals the RHS of (3.4.1) plus $Z_t$, and the projection of $Z_t$ on the span is $0$. QED.