Autoregressive polynomial
= Autoregressive polynomial
{title2=$\Phi(z)=1-\sum_{j=1}^p\phi_jz^j$}
The autoregressive polynomial is $\Phi(z)=1-\sum_{j=1}^p\phi_jz^j$. Substituting the <backshift operator> gives the autoregressive filter. Zeros outside the unit disk give the <causality root criterion for an autoregressive model>; zeros on the unit circle obstruct a nondegenerate stationary innovation-driven solution.