ARIMA Stationarity and Invertibility Conditions

[krassy]

Hello!

A very general question. From various sources, I get different information about the stationarity and invertibility conditions for any ARIMA model.

Consider the following ARMA process:

Yt = a1*Yt-1 + a2*Yt-2 + ... + ap*Yt-p + Et + m1*Et-1 + m2*Et-2 + ... + mq*Et-q

Are the stationarity and invertibility conditions such that |ai| < 1 and |mi| < 1, or Sum(ai) < 1 and Sum(mi) < 1?

Thanks for your help!