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The Algebraic Geometry of Perfect and Sequential Equilibrium

 
Paper:ewp-game/9309001
From:    (William Zame) 
Date: Thu, 30 Sep 93 15:45:32 EDT

Abstract:
Abstract: Two of the most important refinements of the Nash equilibrium concept for extensive form games with perfect recall are Selten's (1975) {\it perfect equilibrium\/} and Kreps and Wilson's (1982) more inclusive {\it sequential equilibrium\/}. These two equilibrium refinements are motivated in very different ways. Nonetheless, as Kreps and Wilson (1982, Section 7) point out, the two concepts lead to similar prescriptions for equilibrium play. For each particular game form, every perfect equilibrium is sequential. Moreover, for almost all assignments of payoffs to outcomes, almost all sequential equilibrium strategy profiles are perfect equilibrium profiles, and all sequential equilibrium outcomes are perfect equilibrium outcomes. \par We establish a stronger result: For almost all assignments of payoffs to outcomes, the sets of sequential and perfect equilibrium strategy profiles are identical. In other words, for almost all games each strategy profile which can be supported by beliefs satisfying the rationality requirement of sequential equilibrium can actually be supported by beliefs satisfying the stronger rationality requirement of perfect equilibrium. \par We obtain this result by exploiting the algebraic/geometric structure of these equilibrium correspondences, following from the fact that they are {\em semi-algebraic sets\/}; i.e., they are defined by finite systems of polynomial inequalities. That the perfect and sequential

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EconWPA began as a conversation between Bob Parks and Larry Blume on January 28, 1993. I located Paul Ginsparg's archive (then xxx.lanl.gov) and he graciously installed his software on a Sun Sparc system which was supporting the department of economics email and computation. EconWPA began accepting papers July 1, 1993 and had ftp, email, gopher and web interfaces. The web interface for submissions was engineered into existence in July 1995. A complete and catastrophic machine failure in 1999 caused the loss of EconWPA's email new paper announcment service at which time there were over 15,000 subscriptions with over 8,000 unique email addresses.

In 2005, Arts and Sciences commandeered the computing services that I had provided to the Department of Economics since 1987. Some might say that the department was sold out, others would (erroneously) claim that centralization is efficient, and still others would claim that I have few marketing skills.

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A Chinese curse states May you live in intersting times. I have. Bob Parks - Jan 2006