Unexploitable games and unbeatable strategies

TL;DR

This paper introduces unexploitable games, a class of symmetric multi-player games, and proves the existence of unbeatable imitation and zero-determinant strategies in these games, extending previous two-player results.

Contribution

It extends the concept of unbeatable strategies from two-player to multi-player symmetric games through the introduction of unexploitable games.

Findings

Existence of unbeatable imitation strategies in unexploitable games.

Existence of unbeatable zero-determinant strategies in non-trivial unexploitable games.

Demonstration of these strategies in the public goods game.

Abstract

Imitation is simple behavior which uses successful actions of others in order to deal with one's own problems. Because success of imitation generally depends on whether profit of an imitating agent coincides with those of other agents or not, game theory is suitable for specifying situations where imitation can be successful. One of the concepts describing successfulness of imitation in repeated two-player symmetric games is unbeatability. For infinitely repeated two-player symmetric games, a necessary and sufficient condition for some imitation strategy to be unbeatable was specified. However, situations where imitation can be unbeatable in multi-player games are still not clear. In order to analyze successfulness of imitation in multi-player situations, here we introduce a class of totally symmetric games called unexploitable games, which is a natural extension of two-player symmetric games without exploitation cycles. We then prove that, for infinitely repeated unexploitable games, there exist unbeatable imitation strategies. Furthermore, we also prove that, for infinitely repeated non-trivial unexploitable games, there exist unbeatable zero-determinant strategies, which unilaterally enforce some relationships on payoffs of players. These claims are demonstrated in the public goods game, which is the simplest unexploitable game. These results show that there are situations where imitation is unbeatable even in multi-player games.