Existence of Multilateral Nash equilibria for families of games

Abstract

This paper introduces two fundamentally new concepts to game theory: multilateral Nash equilibria and families of games. Starting with non-cooperative games, we show how these notions together seamlessly integrate into and naturally extend the classical theory, and simultaneously enable us to prove a powerful (multilateral) Nash equilibrium existence result with minimal assumptions on the game. Classically, a Nash equilibrium is a global strategy such that whichever player unilaterally deviates from the equilibrium, also reduces his own profit. For a k-lateral Nash equilibrium we now require that whichever group of k players collectively changes their strategies, also reduces all of the deviating players' profits. In this way, we obtain a filtration of equilibria, where the higher-lateral equilibria are less frequent. Furthermore, we derive an existence criterion for multilateral Nash equilibria and demonstrate how it reflects the increasing rarity of higher-lateral equilibria. Additionally, we show that some classical games have higher-lateral Nash equilibria, which in every case reveal the structure of these games from a new point of view. A family of games is a parameterized collection of non-cooperative games, where the parameter affects every aspect of the game. Typically, we assume that this dependence is continuous, thereby introducing a new structure. That way, we can avoid analyzing the games one at a time, and instead treat the family as a whole. This allows the parameter to take a central role in our theory, and shifts our attention from seeking a special strategy to searching for a special game with preferred strategies. Our main result proves the existence of a multilateral equilibrium in a family of games, maintaining minimalistic assumptions on the games individually. Surprisingly, the clique covering number of the Kneser graph makes a central appearance.