Abstract: In the problem of allocating a single non-disposable commodity among agents whose preferences are single-peaked, we study a weakening of strategy-proofness called not obvious manipulability (NOM). If agents are cognitively limited, then NOM is sufficient to describe their strategic behavior. We characterize a large family of own-peak-only rules that satisfy efficiency, NOM, and a minimal fairness condition. We call these rules "simple". In economies with excess demand, simple rules fully satiate agents whose peak amount is less than or equal to equal division and assign, to each remaining agent, an amount between equal division and his peak. In economies with excess supply, simple rules are defined symmetrically. These rules can be thought of as a two-step procedure that involves solving a claims problem. We also show that the single-plateaued domain is maximal for the characterizing properties of simple rules. Therefore, even though replacing strategy-proofness with NOM greatly expands the family of admissible rules, the maximal domain of preferences involved remains basically unaltered.

Abstract: In the problem of fully allocating an infinitely divisible commodity among agents whose preferences are single-peaked, we show that the uniform rule is the only allocation rule that satisfies efficiency, the equal division guarantee, consistency, and non-obvious manipulability.

Abstract: Given a standard myopic process in a coalition formation game, an absorbing set is a minimal collection of coalition structures that is never left once entered through this process. Absorbing sets are an important solution concept in coalition formation games, but they have drawbacks: they can be large and hard to obtain. In this paper, we characterize an absorbing set in terms of a collection consisting of a small number of sets of coalitions that we refer to as a “reduced form” of a game. We apply our characterization to study convergence to stability in several economic environments.

Previous version: "Non-convergence to stability in coalition formation games", link to RedNIE Documento de Trabajo 23. September 2020.

Abstract: By endowing the class of tops-only and efficient social choice rules with a dual order structure that exploits the trade-off between different degrees of manipulability and dictatorial power rules allow agents to have, we provide a proof of the Gibbard-Satterthwaite Theorem.

Abstract: In a one-commodity economy with single-peaked preferences and individual endowments, we study different ways in which reallocation rules can be strategically distorted by affecting the set of active agents. We introduce and characterize the family of iterative reallocation rules and show that each rule in this class is withdrawal-proof and endowments-merging-proof, at least one is endowments-splitting-proof, and that no such rule is pre-delivery-proof.

Abstract: In a many-to-one matching model with responsive preferences in which indifferences are allowed, we study three notions of core, three notions of stability, and their relationships. We show that (i) the core contains the stable set, (ii) the strong core coincides with the strongly stable set, and (iii) the super core coincides with the super stable set. We also show how the core and the strong core in markets with indifferences relate to the stable matchings of their associated tie-breaking strict markets.

Abstract: We study envy-free allocations in a many-to-many matching model with contracts in which agents on one side of the market (doctors) are endowed with substitutable choice functions and agents on the other side of the market (hospitals) are endowed with responsive preferences. Envy-freeness is a weakening of stability that allows blocking contracts involving a hospital with a vacant position and a doctor that does not envy any of the doctors that the hospital currently employs. We show that the set of envy-free allocations has a lattice structure. Furthermore, we define a Tarski operator on this lattice and use it to model a vacancy chain dynamic process by which, starting from any envy-free allocation, a stable one is reached.

Abstract: In a voting problem with a finite set of alternatives to choose from, we study the manipulation of tops-only rules. Since all non-dictatorial (onto) voting rules are manipulable when there are more than two alternatives and all preferences are allowed, we look for rules in which manipulations are not obvious. First, we show that a rule does not have obvious manipulations if and only if when an agent vetoes an alternative it can do so with any preference that does not have such alternative in the top. Second, we focus on two classes of tops-only rules: (i) (generalized) median voter schemes, and (ii) voting by committees. For each class, we identify which rules do not have obvious manipulations on the universal domain of preferences.

Abstract: We show that if a rule is strategy-proof, unanimous, anonymous and tops-only, then the preferences in its domain have to be local and weakly single-peaked, relative to a family of partial orders obtained from the rule by confronting at most three alternatives with distinct levels of support. Moreover, if this domain is enlarged by adding a non local and weakly single-peaked preference, then the rule becomes manipulable. We also show that local and weak single-peakedness constitutes a weakening of known and well-studied restricted domains of preferences.

Abstract: In a many-to-one matching model in which firms' preferences satisfy substitutability, we study the set of worker-quasi-stable matchings. Worker-quasi-stability is a relaxation of stability that allows blocking pairs involving a firm and an unemployed worker. We show that this set has a lattice structure and define a Tarski operator on this lattice that models a re-equilibration process and has the set of stable matchings as its fixed points.

Abstract: In a many-to-many matching model in which agents’ preferences satisfy substitutability and the law of aggregate demand, we present an algorithm to compute the full set of stable matchings. This algorithm relies on the idea of “cycles in preferences” and generalizes the algorithm presented in Roth and Sotomayor (1990) for the one-to-one model.

Abstract: As was pointed out to us by Huaxia Zeng, Theorem 1 in Bonifacio and Massó (2020), is not correct. In this note we recall former Theorem 1, exhibit a counterexample of its statement, identify the mistake in its faulty proof, and state and prove the new version of Theorem 1. At the end we give an alternative proof of Lemma 9, whose former proof used incorrectly Lemma 5. 

Abstract: We study social choice rules defined on the domain of semilattice single-peaked preferences. Semilattice single-peakedness has been identified as the necessary condition that a set of preferences must satisfy so that the set can be the domain of a strategy-proof, tops-only, anonymous and unanimous rule. We characterize the class of all such rules on that domain and show that they are deeply related to the supremum of the underlying semilattice structure.

Abstract: We study reallocation rules in the context of a one-good economy consisting of agents with single-peaked preferences and individual endowments. A rule is bribe-proof if no group of agents can compensate one of its subgroups to misrepresent their characteristics (preferences or endowments) in order that each agent is better off after an appropriate redistribution of what the rule reallocates to the group, adjusted by the resource surplus or deficit they all engage in by misreporting endowments. First, we characterize all bribe-proof rules as the class of efficient, (preference and endowment) strategy-proof and weakly replacement monotonic rules, extending the result due to Massó and Neme (Games Econ Behav 61: 331–343, 2007) to our broader framework. Second, we present a full description of the family of bribe-proof rules that in addition are individually rational and peak-only. Finally, we provide two further characterizations of the uniform reallocation rule involving bribe-proofness.

Abstract: In this paper we prove existence theorems of best proximity pairs in uniformly convex spaces, using a fixed point theorem for Kakutani factorizable multi-functions.