The complexity of computing a Nash equilibrium

Abstract
How long does it take until economic agents converge to an equilibrium? By studying the complexity of the problem of computing a mixed Nash equilibrium in a game, we provide evidence that there are games in which convergence to such an equilibrium takes prohibitively long. Traditionally, com- putational problems fall into two classes: those that have a polynomial-time algorithm, and those that are NP-hard. However, the concept of NP-hardness cannot be applied to the rare problems where \every instance has a solution"|for example, in the case of games Nash's theorem asserts that every game has a mixed equilibrium (now known as the Nash equilibrium, in honor of that result). We show that nding a Nash equilibrium is complete for a class of problems called PPAD, containing several other known hard problems; all problems in PPAD share the same style of proof that every instance has a solution.

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