We provide moment inequalities and sufficient conditions for the quick conv
ergence for Markov random walks, without the assumption of uniform ergodici
ty for the underlying Markov chain. Our approach is based on martingales as
sociated with the Poisson equation and Wald equations for the second moment
with a variance formula. These results are applied to nonlinear renewal th
eory for Markov random walks. A random coefficient autoregression model is
investigated as an example. (C) 2000 Elsevier Science B.V. All rights reser
ved. MSC: primary 60G40; secondary 60J10.