A partial proof of Nash's Theorem via exchangeable equilibria

TL;DR

This paper introduces exchangeable equilibria as a new concept to prove the existence of Nash equilibria in finite games without fixed point theorems, but contains an identified error in the proof.

Contribution

It proposes exchangeable equilibria as an innovative intermediate concept to establish Nash equilibrium existence without traditional fixed point methods.

Findings

Exchangeable equilibria exist and approximate Nash equilibria in the limit.

Symmetric games have symmetric Nash equilibria proven via this approach.

The original proof contains an uncorrected error, but many results remain valid.

Abstract

This document consists of two parts: the second part was submitted earlier as a new proof of Nash's theorem, and the first part is a note explaining a problem found in that proof. We are indebted to Sergiu Hart and Eran Shmaya for their careful study which led to their simultaneous discovery of this error. So far the error has not been fixed, but many of the results and techniques of the paper remain valid, so we will continue to make it available online. Abstract for the original paper: We give a novel proof of the existence of Nash equilibria in all finite games without using fixed point theorems or path following arguments. Our approach relies on a new notion intermediate between Nash and correlated equilibria called exchangeable equilibria, which are correlated equilibria with certain symmetry and factorization properties. We prove these exist by a duality argument, using Hart and Schmeidler's proof of correlated equilibrium existence as a first step. In an appropriate limit exchangeable equilibria converge to the convex hull of Nash equilibria, proving that these exist as well. Exchangeable equilibria are defined in terms of symmetries of the game, so this method automatically proves the stronger statement that a symmetric game has a symmetric Nash equilibrium. The case without symmetries follows by a symmetrization argument.