where is y_t when is an argx model stationary where is etau_t in armax model when an ar model is stationary why an arma process is stationary when an armax model is stationary when is an armax system stationary when an armax model is stationary when an armax model is stationary when an ARMAX model is stationary? why stationarity or invertibility is needed why is an armax why is it necessary to invert a process when an armax model is stationary when an armax model is stationary when a ARMA model is stationary? why invertible when an armax model is stationary when an armax model is stationary when an armax model is stationary when an armax model is stationary when an armax model is stationary