Rational Behaviour and Strategy Construction in Infinite Multiplayer Games

. We study infinite games played by arbitrarily many,players on a directed graph. Equilibrium states capture rational behaviour in these games. Instead of the well-known notion of a Nash equilibrium, we focus on the notion of a subgame,perfect equilibrium. We argue that the latter one is more appropriate for the kind of games we study, and we show the existence of a subgame,perfect equilibrium in any infinite game with ω-regular winning,conditions. As, in general, equilibria are not unique, it is appealing to compute one with a maximal,payoff. This problem corresponds naturally to the problem of deciding given a game and two payoff vectors whether the game has an equilibrium with a payoff in between the given thresholds. We show that this problem is decidable for games,with ω-regular winning,conditions played on a finite graph and analyse its complexity. Moreover, we establish that any subgame,perfect equilibrium of a game with ω-regular winning conditions played on a finite graph can be implemented,by finite-state strategies. Finally, we consider logical definability. We state that if we fix the number of players together with an ω-regular winning,condition for each of them and two payoff vectors the property that a game,has a subgame,perfect equilibrium with a payoff in between the given thresholds is definable in the modal µ-calculus.