Best-Effort Strategies for Losing States

Abstract: We consider games played on finite graphs, whose goal is to obtain a trace belonging to a given set of winning traces. We focus on those states from which Player 1 cannot force a win. We explore and compare several criteria for establishing what is the preferable behavior of Player 1 from those states. Along the way, we prove several results of theoretical and practical interest, such as a characterization of admissible strategies, which also provides a simple algorithm for computing such strategies for various common goals, and the equivalence between the existence of positional winning strategies and the existence of positional subgame perfect strategies.