Fix finite pure strategy sets S1 , … , Sn , and let S= S1 ×⋯× Sn . In our model of a random game the agents' payoffs are statistically independent, with each agent's payoff uniformly distributed on the unit sphere in R -super-S. For given nonempty T1 ⊂ S1 , … , Tn ⊂ Sn we give a computationally implementable formula for the mean number of Nash equilibria in which each agent i's mixed strategy has support T i . The formula is the product of two expressions. The first is the expected number of totally mixed equilibria for the truncated game obtained by eliminating pure strategies outside the sets T i . The second may be construed as the "probability" that such an equilibrium remains an equilibrium when the strategies in the sets Si ∖ Ti become available. Copyright The Econometric Society 2005.

An action in an extensive game that has a suboptimal payoff in every sequential equilibrium is said to be useless. There are sequential equilibria in which beliefs at each information set assign positive probability only to those nodes reached by the fewest useless actions. An action is second order useless if it is not useless but is strictly suboptimal in every equilibrium satisfying this condition on beliefs, and there exist sequential equilibria in which beliefs assign positive probability only to those nodes reached by the fewest useless actions, and, in this set, only those nodes requiring the fewest second order useless actions. Higher order uselessness is defined inductively, and beliefs satisfying the associated sequence of conditions are said to be justifiable. The existence of sequential equilibria with justifiable beliefs is demonstrated.

An allocation for an exchange economy with smooth preferences is shown to be Walrasian if there is a set of net trades that is closed under addition, contains the negations of net trades that would improve any agent's final bundle, and is such that each agent's final bundle is weakly preferred to the sum of the initial endowment and any allowed net trade. These conditions characterize the sets of net trades available in equilibria of market games in which randomly paired agents bargain repeatedly and imply that steady state equilibria are Walrasian.

Consider nonempty finite pure strategy sets S1,…,Sn, let S=S1×⋅⋅⋅×Sn, let Ω be a finite space of “outcomes,” let Δ(Ω) be the set of probability distributions on Ω, and let θ: S→Δ(Ω) be a function. We study the conjecture that for any utility in a generic set of n-tuples of utilities on Ω there are finitely many distributions on Ω induced by the Nash equilibria of the game given by the induced utilities on S. We give a counterexample refuting the conjecture for n≥3. Several special cases of the conjecture follow from well known theorems, and we provide some generalizations of these results.

We provide a pure Nash equilibrium existence theorem for games with discontinuous payoffs whose hypotheses are in a number of ways weaker than those of the theorem of Reny (1999). In comparison with Reny's argument, our proof is brief. Our result subsumes a prior existence result of Reny (1999) that is not covered by his theorem. We use the main result to prove the existence of pure Nash equilibrium in a class of finite games in which agents' pure strategies are subsets of a given set, and in turn use this to prove the existence of stable configurations for games, similar to those used by Schelling (1971, 1972) to study residential segregation, in which agents choose locations.