Strategic Behavior in Non-Atomic Games

Typically, economic situations featuring a large number of agents are not modelled with a finite normal form game, rather by a non-atomic game. Consequently, the possibility of strategic interaction may be completely ignored. In order to restore strategic interaction among agents we propose a refinement of Nash equilibrium, strategic equilibrium, for non-atomic games with a continuum of agents, each of whose payo® depends on what he chooses and a societal choice. Given a non-atomic game, we consider a perturbed game in which every player believes that he alone has a small, but positive, impact on the societal choice. A strategy profile is a strategic equilibrium if it is a limit point of a sequence of Nash equilibria of games in which each player's belief about his impact on the societal choice goes to zero. After proving the existence of strategic equilibria, we show that every strategic equilibrium must be a Nash equilibrium of the original non-atomic game, thus, our concept of strategic equilibrium is indeed a refinement of Nash equilibrium. Next, we show that the concept of strategic equilibrium is the natural extension of Nash equilibrium infinite normal form games, to non-atomic games: That is, given any finite normal form game, we consider its non- atomic version, and prove that a strategy profile, in the non-atomic version of the given finite normal form game, is a strategic equilibrium if and only if the associated strategy profile in the finite form game is a Nash equilibrium. Finally, applications of strategic equilibrium is presented examples in which the set of strategic equilibria, in contrast with the set of Nash equilibria, does not contain any implausible Nash equilibrium strategy profiles. These examples are: a game of proportional voting, a game of allocation of public resources, and finally non-atomic Cournot oligopoly

Supermodular social games

A social game is a generalization of a strategic-form game, in which not only the payoff of each player depends upon the strategies chosen by their opponents, but also their set of admissible strategies. Debreu (1952) proves the existence of a Nash equilibrium in social games with continuous strategy spaces. Recently, Polowczuk and Radzik (2004) have proposed a discrete counterpart of Debreu's theorem for two-person social games satisfying some ``convexity properties'. In this note, we define the class of supermodular social games and give an existence theorem for this class of games.Strategic-form games, social games, supermodularity, Nash equilibrium, existence.

Supermodular Social Games

A social game is a generalization of a strategic-form game, in which not only the payoff of each player depends upon the strategies chosen by their opponents, but also their set of admissible strategies. Debreu (1952) proves the existence of a Nash equilibrium in social games with continuous strategy spaces. Recently, Polowczuk and Radzik (2004) have proposed a discrete counterpart of Debreu's theorem for two-person social games satisfying some ''convexity properties''. In this note, we define the class of supermodular social games and give an existence theorem for this class of games.strategic-form games, social games, supermodularity, Nash equilibrium, existence

Periodic Strategies: A New Solution Concept and an Algorithm for NonTrivial Strategic Form Games

We introduce a new solution concept, called periodicity, for selecting optimal strategies in strategic form games. This periodicity solution concept yields new insight into non-trivial games. In mixed strategy strategic form games, periodic solutions yield values for the utility function of each player that are equal to the Nash equilibrium ones. In contrast to the Nash strategies, here the payoffs of each player are robust against what the opponent plays. Sometimes, periodicity strategies yield higher utilities, and sometimes the Nash strategies do, but often the utilities of these two strategies coincide. We formally define and study periodic strategies in two player perfect information strategic form games with pure strategies and we prove that every non-trivial finite game has at least one periodic strategy, with non-trivial meaning non-degenerate payoffs. In some classes of games where mixed strategies are used, we identify quantitative features. Particularly interesting are the implications for collective action games, since there the collective action strategy can be incorporated in a purely non-cooperative context. Moreover, we address the periodicity issue when the players have a continuum set of strategies available.Comment: Revised version, similar to the one published in Advances in Complex System

Computational Extensive-Form Games

We define solution concepts appropriate for computationally bounded players playing a fixed finite game. To do so, we need to define what it means for a \emph{computational game}, which is a sequence of games that get larger in some appropriate sense, to represent a single finite underlying extensive-form game. Roughly speaking, we require all the games in the sequence to have essentially the same structure as the underlying game, except that two histories that are indistinguishable (i.e., in the same information set) in the underlying game may correspond to histories that are only computationally indistinguishable in the computational game. We define a computational version of both Nash equilibrium and sequential equilibrium for computational games, and show that every Nash (resp., sequential) equilibrium in the underlying game corresponds to a computational Nash (resp., sequential) equilibrium in the computational game. One advantage of our approach is that if a cryptographic protocol represents an abstract game, then we can analyze its strategic behavior in the abstract game, and thus separate the cryptographic analysis of the protocol from the strategic analysis

A conjectural cooperative equilibrium for strategic form games

This paper presents a new cooperative equilibrium for strategic form games, denoted Conjectural Cooperative Equilibrium (CCE). This concept is based on the expectation that joint deviations from any strategy profile are followed by an optimal and noncooperative reaction of non deviators. We show that CCE exist for all symmetric supermodular games. Furthermore, we discuss the existence of a CCE in specific submodular games employed in the literature on environmental agreements.Strong Nash Equilibrium, Cooperative Games, Public Goods

Feature-based Choice and Similarity in Normal-form Games: An Experimental Study

In this paper, we test the effect of descriptive "features" on initial strategic behavior in normal form games, where the term "descriptive" indicates all those features which can be modified without altering the (Nash) equilibrium structure of a game. Our experimental subjects behaved according to some simple heuristics based on descriptive features, and we observed that these heuristics were stable even across strategically different games. These findings indicate the need to incorporate descriptive features into models describing strategic sophistication in normal form games. Analysis of choice patterns and individual behavior indicates that non-equilibrium choices may derive from incorrect and simplified mental representations of the game structure, rather than from beliefs in other players' irrationality. We suggest how level-k and cognitive hierarchy models might be extended to account for heuristic-based and feature-based behavior.normal form games, one-shot games, response times, dominance, similarity, categorization, focal points, individual behavior

Non-cooperative Games

Non-cooperative games are mathematical models of interactive strategic decision situations.In contrast to cooperative models, they build on the assumption that all possibilities for commitment and contract have been incorporated in the rules of the game.This contribution describes the main models (games in normal form, and games in extensive form), as well as the main concepts that have been proposed to solve these games.Solution concepts predict the outcomes that might arise when the game is played by "rational" individuals, or after learning processes have converged.Most of these solution concepts are variations of the equilibrium concept that was proposed by John Nash in the 1950s, a Nash equilibrium being a combination of strategies such that no player can improve his payoff by deviating unilaterally.The paper also discusses the justifications of these concepts and concludes with remarks about the applicability of game theory in contexts where players are less than fully rational.noncooperative games

Transfers, Contracts and Strategic Games

This paper analyses the role of transfer payments and strategic con- tracting within two-person strategic form games with monetary pay- offs. First, it introduces the notion of transfer equilibrium as a strat- egy combination for which individual stability can be supported by allowing the possibility of transfers of the induced payoffs. Clearly, Nash equilibria are transfer equilibria, but under common regularity conditions the reverse is also true. This result typically does not hold for finite games without the possibility of randomisation, and transfer equilibria for this particular class are studied in some detail. The second part of the paper introduces, also within the setting of finite games, contracting on monetary transfers as an explicit strategic option, resulting in an associated two-stage contract game. In the first stage of the contract game each player has the option of proposing transfer schemes for an arbitrary collection of outcomes. Only if the players fully agree on the entire set of transfer proposals, the payoffs of the game to be played in the second stage are modified accordingly. The main results provide explicit characterisations of the sets of payoff vectors that are supported by Nash equilibrium and virtual subgame perfect equilibrium, respectively.monetary transfer scheme;transfer equilibrium;contract game;virtual subgame perfect equilibrium;Folk theorems