On structural properties of optimal average cost functions in Markov decision processes with Borel spaces and universally measurable policies

We consider Markov decision processes (MDPs) with Borel state and action spaces and universally measurable policies. For several long-run average cost criteria and two classes of MDPs, we prove sufficient conditions for the optimal average cost functions to be constant almost everywhere with respect to certain sigma-finite measures. Besides suitable boundedness conditions on the positive parts of the one-stage costs, the key condition here is that each subset of states with positive measure be reachable with probability one under some policy. Our proofs exploit an inequality for the optimal average cost functions and its connection with submartingales, and, in a special case that involves stationary policies, also use the theory of recurrent Markov chains. (c) 2021 Elsevier Inc. All rights reserved.