The ex ante incentive compatible core of an exchange economy with and without indivisibilities.

Le coeur ex ante incitatif d'une économie d'échange en information incomplète est le coeur, au sens de la théorie des jeux coopératifs standard, d'une fonction caractéristique qui rend compte du fait que les coalitions allouent les biens grâce à des mécanismes aléatoires incitatifs. Nous passons d'abord en revue quelques résultats dans le cas de biens parfaitement divisibles. Des exemples montrent alors que coeur ex ante incitatif peut être vide, même si les fonctions d'utilité sont quasi-linéaires. Si, en plus de la quasi-linéarité, on formule des hypothèses supplémentaires, telles que les valeurs privées et indépendantes, on peut établir la non-vacuité du coeur en construisant des mécanismes incitatifs efficaces ex post, suivant la méthode de d'Aspremont et Gérard-Varet. Nous introduisons également de l' information incomplète dans les économies avec biens divisibles de Shapley et Scarf et nous démontrons que le coeur ex ante incitatif est toujours non-vide dans ce cadre.The ex ante incentive compatible core of an exchange economy with private information is the (standard) core of a characteristic function which expresses the fact that coalitions allocate goods by means of random incentive compatible mechanisms. We first survey some results in the case of perfectly divisible goods. Examples then show that the ex ante incentive compatible core can be empty, even if utility functions are quasi-linear. If, in addition to quasi-linearity, further assumptions are made (like independent private values), the non-emptiness of the core follows nevertheless from d'Aspremont and Gérard-Varet's construction of incentive compatible, ex post efficient mechanisms. We also introduce a private information version of Shapley and Scarf's economies with indivisible goods, and prove that the ex ante incentive compatible core is always non-empty in this framework.Information incomplète; Mécanisme incitatif; Biens indivisibles;

Correlated equilibria and communication in games.

Analyse bayésienne; Théorie des jeux; Information privée;

Communication in Bayesian games: Overview of work on implementing mediators in game theory.

game theory; Bayesian games;

Feasible mechanisms in economies with type-dependent endowments.

We propose two classes of allocation games for N.T.U. and T.U. exchange economies in which initial endowments and preferences depend on the agents’ private information. In both models, agents make non-verifiable claims about their types and effective deposits of consumption goods, which are redistributed by the planner. In a W-allocation game, the agents can withhold part of their endowment, namely consume whatever they do not deposit. In a D-allocation game, the agents can just destroypart of their endowment. W- and D- incentive compatible (I.C.) direct allocation mechanisms ask every agent to reveal his type and to make a deposit consistent with his reported type. The revelation principle holds in full generality for D-I.C. mechanisms but some care is needed for W-I.C. mechanisms. We further investigate the properties of both classes of mechanisms under common assumptions like non-exclusive information and/or constant aggregate endowment. In T.U. economies, W-I.C. and D-I.C. mechanisms are ex ante equivalent.Théorie des jeux;

Core-stable rings in second price auctions with common values.

In a commonvalueauction in which the information partitions of the bidders are connected, all rings are core-stable. More precisely, the ex ante expected utilities of rings, at the (noncooperative) sophisticated equilibrium proposed by Einy et al. [Einy, E., Haimanko, O., Orzach, R., Sela, A., 2002. Dominance solvability of second-pricesauctions with differential information. Journal of Mathematical Economics 37, 247–258], describe a cooperative games in characteristic function form, in spite of the underlying strategic externalities. A ring is core-stable if the core of this characteristic function is not empty. Furthermore, every ring can implement its sophisticated equilibrium strategy by means of an incentive compatible mechanism. An example shows that, if the bidders’ information partitions are not connected, rings may no longer be core-stable.Characteristic function; Partition form game; Core; Collusion; Bayesian game; Auctions;

Core-stable Rings in Second Price Auctions with Common Values

In a common value auction in which the information partitions of the bidders are connected, all rings are core-stable. More precisely, the ex ante expected utilities of rings, at the (noncooperative) sophisticated equilibrium proposed by Einy, Haimanko, Orzach and Sela (Journal of Mathematical Economics, 2002), describe a cooperative game, in characteristic function form, in spite of the underlying strategic externalities. A ring is core-stable if the core of this characteristic function is not empty. Furthermore, every ring can implement its sophisticated equilibrium strategy by means of an incentive compatible mechanism.Auctions, Bayesian Game, Collusion, Core, Partition Form Game, Characteristic Function

Core-stable rings in auctions with independent private values.

We propose a semi-cooperative game theoretic approach to check whether a given coalition is stable in a Bayesian game with independent private values. The ex ante expected utilities of coalitions, at an incentive compatible (noncooperative) coalitional equilibrium, describe a (cooperative) partition form game. A coalition is core-stable if the core of a suitable characteristic function, derived from the partition form game, is not empty. As an application, we study collusion in auctions in which the bidders' final utility possibly depends on the winner's identity. We show that such direct externalities offer a possible explanation for cartels'structures (not) observed in practice.Core; partition function game; Collusion; Auctions; Bayesian game;

Essential Data, Budget Sets and Rationalization

According to a minimalist version of Afriat’s theorem, a consumer behaves as a utility maximizer if and only if a feasibility matrix associated with his choices is cyclically consistent. An ”essential experiment” consists of observed consumption bundles (x1,xn) and a feasibility matrix α. Starting with a standard experiment, in which the economist has specific budget sets in mind, we show that the necessary and sufficient condition for the existence of a utility function rationalizing the experiment, namely, the cyclical consistency of the associated feasibility matrix, is equivalent to the existence, for any budget sets compatible with the deduced essential experiment, of a utility function rationalizing them (and typically depending on them). In other words, the conclusion of the standard rationalizability test, in which the economist takes budget sets for granted, does not depend on the full specification of the underlying budget sets but only on the essential data that these budget sets generate. Starting with an essential experiment (x1,...,xn;α), we show that the cyclical consistency of α, together with a further consistency condition involving both (x1,...,xn) and α, guarantees that the essential experiment is rationalizable almost robustly, in the sense that there exists a single utility function which rationalizes at once almost all budget sets which are compatible with (x1,...,xn;α). The conditions are also trivially necessary.Afriat’s theorem, budget sets, cyclical consistency, rational choice, revealed preference

Communication equilibria with partially verifiable types.

This paper studies the set of equilibria that can be achieved by adding general communication systems to Bayesian games in which some information can be certified or, equivalently, in which players’ types are partially verifiable. Certifiability of information is formalized by a set of available reports for each player that varies with the true state of the world. Given these state-dependent sets of reports, we characterize canonical equilibria for which generalized versions of the revelation principle are valid. Communication equilibria and associated canonical representations are obtained as special cases when no information can be certified.Information privée; Théorie des jeux; Analyse bayésienne;

Sender-receiver games with cooperation

International audienceWe consider generalized sender–receiver games in which the sender also has an action to choose, but this action is payoff-relevant only to himself. We study “cooperate and talk” equilibria (CTE) in which, before sending his message, the sender can commit to delegate his decision to the receiver. CTE are beneficial to the receiver (with respect to no communication) and unlike the equilibria of the plain cheap talk game, preserve him from afterwards regret. While existence of CTE cannot be guaranteed in general, a sufficient condition is that the receiver has a “uniform punishment decision” against the sender