Optimality inequalities for average cost Markov decision processes and the optimality of (s,S) policies

For general state and action space Markov decision processes, we present sufficient conditions for the existence of solutions of the average cost optimality inequalities. These conditions also imply the convergence of both the optimal discounted cost value function and policies to the corresponding objects for the average costs per unit time case. Inventory models are natural applications of our results. We describe structural properties of average cost optimal policies for the cash balance problem; an inventory control problem where the demand may be negative and the decision-maker can produce or scrap inventory. We also show the convergence of optimal thresholds in the finite horizon case to those under the expected discounted cost criterion and those under the expected discounted costs to those under the average costs per unit time criterion.