Hardness of Independent Learning and Sparse Equilibrium Computation in Markov Games

We consider the problem of decentralized multi-agent reinforcement learning in Markov games. A fundamental question is whether there exist algorithms that, when adopted by all agents and run independently in a decentralized fashion, lead to no-regret for each player, analogous to celebrated convergence results for no-regret learning in normal-form games. While recent work has shown that such algorithms exist for restricted settings (notably, when regret is defined with respect to deviations to Markovian policies), the question of whether independent no-regret learning can be achieved in the standard Markov game framework was open. We provide a decisive negative resolution this problem, both from a computational and statistical perspective. We show that: 1. Under the widely-believed complexity-theoretic assumption that PPAD-hard problems cannot be solved in polynomial time, there is no polynomial-time algorithm that attains noregret in general-sum Markov games when executed independently by all players, even when the game is known to the algorithm designer and the number of players is a small constant. 2. When the game is unknown, no algorithm-regardless of computational efficiency-can achieve no-regret without observing a number of episodes that is exponential in the number of players. Perhaps surprisingly, our lower bounds hold even for seemingly easier setting in which all agents are controlled by a a centralized algorithm. They are proven via lower bounds for a simpler problem we refer to as SparseCCE, in which the goal is to compute by any meanscentralized, decentralized, or otherwise-a coarse correlated equilibrium that is "sparse" in the sense that it can be represented as a mixture of a small number of "product" policies. The crux of our approach is a novel application of aggregation techniques from online learning [Vov90, CBL06], whereby we show that any algorithm for the SparseCCE problem can be used to compute approximate Nash equilibria for non-zero sum normal-form games; this enables the application of well-known hardness results for Nash.