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Semidefinite Programming and Nash Equilibria in Bimatrix Games

Amir Ali Ahmadi (a_a_a***at***princeton.edu)
Jeffrey Zhang (jeffz***at***princeton.edu)

Abstract: We explore the power of semidefinite programming (SDP) for finding additive epsilon-approximate Nash equilibria in bimatrix games. We introduce an SDP relaxation for a quadratic programming formulation of the Nash equilibrium (NE) problem and provide a number of valid inequalities to improve the quality of the relaxation. If a rank-1 solution to this SDP is found, then an exact NE can be recovered. We show that for a strictly competitive game, our SDP is guaranteed to return a rank-1 solution. Furthermore, we prove that if a rank-2 solution to our SDP is found, then a 5/11-NE can be recovered for any game, or a 1/3-NE for a symmetric game. We propose two algorithms based on iterative linearization of smooth nonconvex objective functions that are designed so that their global minima coincide with rank-1 solutions. Empirically, we demonstrate that these algorithms often recover solutions of rank at most two and epsilon close to zero. We then show how our SDP approach can address two (NP-hard) problems of economic interest: finding the maximum welfare achievable under any NE, and testing whether there exists a NE where a particular set of strategies is not played. Finally, we show the connection between our SDP and the first level of the Lasserre/sum of squares hierarchy.

Keywords: Semidefinite optimization, Game theory, Approximate Nash equilibria

Category 1: Linear, Cone and Semidefinite Programming (Semi-definite Programming )

Category 2: Other Topics (Game Theory )

Citation: Submitted for publication

Download: [PDF]

Entry Submitted: 06/26/2017
Entry Accepted: 06/26/2017
Entry Last Modified: 02/26/2018

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