Equilibrium Computation For Knockout Tournaments Played By Groups.

In single-elimination knockout tournaments, participants face each other based on a starting seeding and progress to the next rounds by beating their direct opponents. In this paper we initiate the study of coalitional knockout tournaments, which generalise single-elimination knockout tournaments by allowing groups of players, or coalitions, to strategically select one of their members to take part in the tournament, following the starting seeding. We investigate the algorithmic properties of pure strategies Nash equilibria in these games under various setups, i.e., whether or not choices can be made at each round and whether or not tournament progression is important to the group. Despite the more complex tournament structure when compared to single-elimination, we provide (quasi-)polynomial-time algorithms for all cases. Our results can be applied to those tournaments where pre-play selection plays an important role, such as sport events or elections with run-off.