 Common factor cancellation in an ARMA model

An ARMA equation need not be a minimal-order representation when its autoregressive polynomial and moving-average polynomial have a common factor. For a stationary causal solution, a common stable factor can be cancelled using its convergent inverse filter. For example, $(1-0.5B)X=(1-0.5B)(1-0.9B)W$ reduces to $X=(1-0.9B)W$, an MA(1). Its autocorrelation function vanishes beyond lag one, despite the displayed ARMA(1,2) equation. Root conditions should be applied to a reduced representation when asserting necessary and sufficient causality and invertibility root criteria for an ARMA model.