Galton-Watson games

Abstract

We consider two-player combinatorial games in which the graph of positions is random and perhaps infinite, focusing on directed Galton-Watson trees. As the offspring distribution is varied, a game can undergo a phase transition, in which the probability of a draw under optimal play becomes positive. We study the nature of the phase transitions which occur for normal play rules (where a player unable to move loses the game) and misere rules (where a player unable to move wins), as well as for an "escape game" in which one player tries to force the game to end while the other tries to prolong it forever. For instance, for a Poisson$(\lambda)$ offspring distribution, the game tree is infinite with positive probability as soon as $\lambda>1$, but the game with normal play has positive probability of draws if and only if $\lambda>e$. The three games generally have different critical points; under certain assumptions the transitions are continuous for the normal and misere games and discontinuous for the escape game, but we also discuss cases where the opposite possibilities occur. We connect the nature of the phase transitions to the behaviour of quantities such as the expected length of the game under optimal play. We also establish inequalities relating the games to each other; for instance, the probability of a draw is at least as great in the misere game as in the normal game.