Mixed autoregressive-moving average multivariate processes with time-dependent coefficients

Conditions for mixed autoregressive-moving average processes with time-dependent coefficients to be purely nondeterministic and invertible can be obtained from classical difference equations theory. These conditions involve one-sided Green's functions or their matricial equivalents. A recursive computation of these functions is proposed, which allows one to drop the assumption of nondegeneracy classicaly made about the highest order matrix of difference operators; it constitutes thus a generalized definition of these functions.