Ambrus A. Coalitional rationalizability. Quarterly Journal of Economics. 2006;121 (3) :903-930. Publisher's VersionAbstract

This paper investigates how groups or coalitions of players can act in their collective interest in non-cooperative normal form games even if equilibrium play is not assumed. The main idea is that each member of a coalition will confine play to a subset of their strategies if it is in their mutual interest to do so. An iterative procedure of restrictions is used to define a non-cooperative solution concept, the set of coalitionally rationalizable strategies. The procedure is analogous to iterative deletion of never best response strategies, but operates on implicit agreements by different coalitions. The solution set is a nonempty subset of the rationalizable strategies.

Ambrus A. Dynamic Coalitional Agreements - coalitional rationalizability in multi-stage games. 2003.Abstract

This paper extends the concept of coalitional rationalizability of Ambrus(01) to incorporate sequential rationality in multi-stage games with observable actions and incomplete information. Agreements among players are implicit, it is assumed that players cannot communicate with each other during the game. They reflect a reasoning procedure which entails restricting strategies in a mutually advantegous way. They can be conditional on observed histories and players' types, which corresponds to allowing players to make agreements ex post and along the course of play. An agreement that is conditioned on a history is evaluated from the point of view of that history. This introduces a dynamic interaction among coalitional agreements with features of both backward and forward induction. Coalitional agreements iteratively define the set of extensive form coalitionally rationalizable strategies. This solution concept has a number of analogous properties with normal form coalitionally rationalizability. It is always nonempty. The set of outcomes consistent with it is a subset of the outcomes consistent with extensive form rationalizability, and it is robust to the order in which agreements are made. In games of perfect information extensive form coalitional rationalizability is outcome equivalent to extensive form rationalizability. Perfect coalition-proof Nash equilibria and renegotiation-proof Nash equilibria do not have to be contained in the solution set, even in two-player games, because those concepts do not imply forward induction reasoning. An alternative notion of extensive form coalitional rationalizability is also provided, assuming that coalitional agreements can only be made ex ante, but sequential individual rationality is maintained.