A nonstandard characterization of sequential equilibrium, perfect equilibrium, and proper equilibrium

A nonstandard characterization of sequential equilibrium, perfect equilibrium, and proper equilibrium

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Article ID: iaor20162544
Volume: 38
Issue: 1
Start Page Number: 37
End Page Number: 49
Publication Date: Mar 2009
Journal: International Journal of Game Theory
Authors:
Keywords: Nash equilibrium, sequential game
Abstract:

New characterizations of sequential equilibrium, perfect equilibrium, and proper equilibrium are provided that use nonstandard probability. It is shown that there exists a belief system μ such that ( σ , μ ) equ1 is a sequential equilibrium in an extensive game with perfect recall iff there exist an infinitesimal ε equ2 and a completely mixed behavioral strategy profile σ′ (so that σ i equ3 assigns positive, although possibly infinitesimal, probability to all actions at every information set) that differs only infinitesimally from σ equ4 such that at each information set I for player i, σ i is an ε equ5 ‐best response to σ i equ6 conditional on having reached I. Note that the characterization of sequential equilibrium does not involve belief systems. There is a similar characterization of perfect equilibrium; the only difference is that σ i must be a best response to σ i equ7 conditional on having reached I. Yet another variant is used to characterize proper equilibrium.

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