Memory Requirements in Non-Zero-Sum Games

The interaction between a system and the components modeling its environment is traditionally modeled by a multi-player game played on a finite graph. In zero-sum games, the players have conflicting objectives, and it is clear that increasing the memory of the environment players can only make it harder for the system to win. In non-zero-sum games, the objectives of the players may overlap. There, typical questions concern the stability of the game and the equilibria the players may reach. In particular, in rational synthesis (RS), the goal is to find an equilibrium that satisfies the objective of the system. We study how the memory of the environment players may affect the existence of an RS solution. As we show, the picture is diverse, even when the objectives of all players are memoryless. On the one hand, when stability amounts to a Nash equilibrium (NE), then increasing the memory of the environment may only help the system to suggest an RS solution. On the other hand, when the notion of stability involves deviations by coalitions of environment players, for example in a strong Nash equilibrium (SNE), then increasing their memory may sometimes enable and sometimes prevent the existence of an RS solution. We study memory bounds for the players, showing that the memory required may be polynomial in an NE-RS solution and exponential in an SNE-RS solution. We also solve the SNE-RS problem, show that it is PSPACE-complete, and relate the differences between NE and SNE with the differences between cooperative and non-cooperative RS.