Computing the Team-maxmin Equilibrium in Single-Team Single-Adversary Team Games

A team game is a non-cooperative normal-form game in which some teams of players play against others. Team members share a common goal but, due to some constraints, they cannot act jointly. A real-world example is the protection of environments or complex infrastructures by different security agencies: they all protect the area with valuable targets but they have to act individually since they cannot share their defending strategies (of course, they are aware of the presence of the other agents). Here, we focus on zero-sum team games with n players, where a team of n - 1 players plays against one single adversary. In these games, the most appropriate solution concept is the Team-maxmin equilibrium, i.e., the Nash equilibrium that ensures the team the highest payoff. We investigate the Team-maxmin equilibrium, characterizing the utility of the team and showing that it can be irrational. The problem of computing such equilibrium is NP-hard and cannot be approximated within a factor of 1/n(c). The exact solution can only be found by global optimization. We propose two approximation algorithms: the former is a modified version of an already existing algorithm, the latter is a novel anytime algorithm. We computationally investigate such algorithms, providing bounds on the utility for the team. We experimentally evaluate the algorithms analyzing their performance w.r.t. a global optimization approach and evaluate the loss due to the impossibility of correlating.