Towards Minimax Optimality of Model-based Robust Reinforcement Learning

We study the sample complexity of obtaining an ϵ-optimal policy in Robust discounted Markov Decision Processes (RMDPs), given only access to a generative model of the nominal kernel. This problem is widely studied in the non-robust case, and it is known that any planning approach applied to an empirical MDP estimated with Õ(^{H3 |}_ϵ^S₂^||A^|) samples provides an ϵ-optimal policy, which is minimax optimal. Results in the robust case are much more scarce. For sa- (resp s-) rectangular uncertainty sets, until recently the best-known sample complexity was Õ(^{H4 |S}_ϵ2^{|2 |}A^|) (resp. Õ(^{H4 |S}_ϵ^|₂^{2 |}A^|2)), for specific algorithms and when the uncertainty set is based on the total variation (TV), the KL or the Chi-square divergences. In this paper, we consider uncertainty sets defined with an L_p-ball (recovering the TV case), and study the sample complexity of any planning algorithm (with high accuracy guarantee on the solution) applied to an empirical RMDP estimated using the generative model. In the general case, we prove a sample complexity of Õ(^{H4 |}_ϵ^S₂^||A^|) for both the sa- and s-rectangular cases (improvements of |S| and |S||A| respectively). When the size of the uncertainty is small enough, we improve the sample complexity to Õ(^{H3 |}_ϵ^S₂^||A^|), recovering the lower-bound for the non-robust case for the first time and a robust lower-bound. Finally, we also introduce simple and efficient algorithms for solving the studied L_p robust MDPs.