# Global Algorithms for Mean-Variance Optimization in Markov Decision Processes

## TL;DR

This paper introduces a novel global algorithm for mean-variance optimization in Markov decision processes, transforming the problem into a bilevel MDP and efficiently finding the optimal policy with proven convergence.

## Contribution

It proposes a new bilevel MDP approach using pseudo mean and variance, enabling the first efficient global optimization algorithm for mean-variance in MDPs.

## Key findings

- Algorithm converges to the global optimum.
- Numerical experiments show high efficiency.
- Applicable to variance minimization as well.

## Abstract

Dynamic optimization of mean and variance in Markov decision processes (MDPs) is a long-standing challenge caused by the failure of dynamic programming. In this paper, we propose a new approach to find the globally optimal policy for combined metrics of steady-state mean and variance in an infinite-horizon undiscounted MDP. By introducing the concepts of pseudo mean and pseudo variance, we convert the original problem to a bilevel MDP problem, where the inner one is a standard MDP optimizing pseudo mean-variance and the outer one is a single parameter selection problem optimizing pseudo mean. We use the sensitivity analysis of MDPs to derive the properties of this bilevel problem. By solving inner standard MDPs for pseudo mean-variance optimization, we can identify worse policy spaces dominated by optimal policies of the pseudo problems. We propose an optimization algorithm which can find the globally optimal policy by repeatedly removing worse policy spaces. The convergence and complexity of the algorithm are studied. Another policy dominance property is also proposed to further improve the algorithm efficiency. Numerical experiments demonstrate the performance and efficiency of our algorithms. To the best of our knowledge, our algorithm is the first that efficiently finds the globally optimal policy of mean-variance optimization in MDPs. These results are also valid for solely minimizing the variance metrics in MDPs.