On continuous ordinal potential games

If the preferences of the players in a strategic game satisfy certain continuity conditions, then the acyclicity of individual improvements implies the existence of a Nash equilibrium. Moreover, starting from any strategy profile, an arbitrary neighborhood of the set of Nash equilibria can be reached after a finite number of individual improvements.potential game; compact-continuous game; finite improvement property

Shapley's "2 by 2" theorem for game forms

If a finite two person game form has the property that every 2-by-2 fragment is Nash consistent, then no derivative game admits an individual improvement cycle.

On the existence of maximal elements: An impossibility theorem

Most properties of binary relations considered in the decision literature can be expressed as the impossibility of certain ``configurations.'' There exists no condition of this form which would hold for a binary relation on a subset of a finite-dimensional Euclidean space if and only if the relation admits a maximal element on every nonempty compact subset of its domain.Binary relation; Maximal element; Necessary and sufficient condition; Potential games

On the choice of most-preferred alternatives

Maximal elements of a binary relation on compact subsets of a metric space define a choice function. Necessary and sufficient conditions are found for: (1) the choice function to have nonempty values and be path independent; (2) the choice function to have nonempty values provided the underlying relation is an interval order. For interval orders and semiorders, the same properties are characterized in terms of representations in a chain.Maximal element; Path independence; Interval order; Semiorder

Monotone comparative statics: Changes in preferences vs changes in the feasible set

Let a preference ordering on a lattice be perturbed. As is well known, single crossing conditions are necessary and sufficient for a monotone reaction of the set of optimal choices from every chain. Actually, there are several interpretations of monotonicity and several corresponding single crossing conditions. We describe restrictions on the preferences that ensure a monotone reaction of the set of optimal choices from every sublattice whenever a perturbation of preferences satisfies the corresponding single crossing condition. Quasisupermodularity is necessary if we want monotonicity in every conceivable sense; otherwise, weaker conditions will do.strategic complementarity; monotone comparative statics; best response correspondence; single crossing; quasisupermodularity

Acyclicity of improvements in finite game forms

Game forms are studied where the acyclicity, in a stronger or weaker sense, of (coalition or individual) improvements is ensured in all derivative games. In every game form generated by an ``ordered voting'' procedure, individual improvements converge to Nash equilibria if the players restrict themselves to ``minimal'' strategy changes. A complete description of game forms where all coalition improvement paths lead to strong equilibria is obtained: they are either dictatorial, or voting (or rather lobbing) about two outcomes. The restriction to minimal strategy changes ensures the convergence of coalition improvements to strong equilibria in every game form generated by a ``voting by veto'' procedure.Improvement dynamics; Game form; Perfect information game; Potential game; Voting by veto

On the existence of most-preferred alternatives in complete lattices

If a preference ordering on a complete lattice is quasisupermodular, or just satisfies a rather weak analog of the condition, then it admits a maximizer on every subcomplete sublattice if and only if it admits a maximizer on every subcomplete subchainlattice optimization; quasisupermodularity

Robert Louis Stevenson's Bottle Imp: A strategic analysis

The background of Stevenson's story is viewed as a stylized model of participation in a financial pyramid. You are invited to participate in a dubious activity. If you refuse, you neither gain nor lose anything. If you accept, you will gain if you are able to find somebody who will take your place on exactly the same conditions, but suffer a loss otherwise. The catch is that there is a finite number of discrete steps at which the substitution can be done, so the agent who joins in at the last step inevitably loses. Clearly, rational agents will not agree to participate at any step. However, an arbitrarily small probability of a "bailout," in which case that last agent will get the same gain as every other participant, plus an appropriately asymmetric structure of private information, change everything, so the proposal will be accepted in a (subgame perfect) equilibrium, provided there are sufficiently many steps ahead

On continuous ordinal potential games

If the preferences of the players in a strategic game satisfy certain continuity conditions, then the acyclicity of individual improvements implies the existence of a Nash equilibrium. Moreover, starting from any strategy profile, an arbitrary neighborhood of the set of Nash equilibria can be reached after a finite number of individual improvements