Settling the complexity of computing two-player Nash equilibria

  • Authors:
  • Xi Chen;Xiaotie Deng;Shang-Hua Teng

  • Affiliations:
  • Tsinghua University, Beijing, China;City University of Hong Kong, Hong Kong, China;Boston University and Akamai Technologies Inc., Boston, Massachusetts

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 2009

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Abstract

We prove that Bimatrix, the problem of finding a Nash equilibrium in a two-player game, is complete for the complexity class PPAD (Polynomial Parity Argument, Directed version) introduced by Papadimitriou in 1991. Our result, building upon the work of Daskalakis et al. [2006a] on the complexity of four-player Nash equilibria, settles a long standing open problem in algorithmic game theory. It also serves as a starting point for a series of results concerning the complexity of two-player Nash equilibria. In particular, we prove the following theorems: —Bimatrix does not have a fully polynomial-time approximation scheme unless every problem in PPAD is solvable in polynomial time. —The smoothed complexity of the classic Lemke-Howson algorithm and, in fact, of any algorithm for Bimatrix is not polynomial unless every problem in PPAD is solvable in randomized polynomial time. Our results also have a complexity implication in mathematical economics: —Arrow-Debreu market equilibria are PPAD-hard to compute.