Robust Equilibria in Concurrent Games

We study the problem of finding robust equilibria in multiplayer concurrent games with mean payoff objectives. A (k, t)-robust equilibrium is a strategy profile such that no coalition of size k can improve the payoff of one its member by deviating, and no coalition of size t can decrease the payoff of other players. We are interested in pure equilibria, that is, solutions that can be implemented using non-randomized strategies. We suggest a general transformation from multiplayer games to two-player games such that pure equilibria in the first game correspond to winning strategies in the second one. We then devise from this transformation, an algorithm which computes equilibria in mean-payoff games. Robust equilibria in mean-payoff games reduce to winning strategies in multidimensional mean-payoff games for some threshold satisfying some constraints. We then show that the existence of such equilibria can be decided in polynomial space, and that the decision problem is PSPACE-complete.