Marina Núñez

Buyer-seller exactness in the assignment game

Abstract. In the assignment game framework, we try to identify those assignment matrices in which no entry can be increased without changing the core of the game. These games will be called buyer-seller exact games and satisfy the condition that each mixed-pair coalition attains the corresponding matrix entry in the core of the game. For a given assignment game, a unique buyer-seller exact assignment game with the same core is proved to exist. In order to identify this matrix and to provide a characterization of those assignment games which are buyer-seller exact in terms of the assignment matrix, attainable upper and lower core bounds for the mixed-pair coalitions are found. As a consequence, an open question posed in Quint (1991) regarding a canonical representation of a “45o-lattice” by means of the core of an assignment game can now be answered.

The assignment game: the τ-value

Abstract. We provide some formulae for the τ-value in the case of the assignment game and prove that it coincides with the midpoint between the buyers-optimal and the sellers-optimal core allocations. As a consequence, the τ-value of an assignment game always lies in the core. Some comparative statics of this solution is analyzed: the pairwise monotonicity and the effect of new entrants.

Characterization of the extreme core allocations of the assignment game

Although assignment games are hardly ever convex, in this paper a characterization of their set or extreme points of the core is provided, which is also valid for the class of convex games. For each ordering in the player set, a payoff vector is defined where each player receives his marginal contribution to a certain reduced game played by his predecessors. We prove that the whole set of reduced marginal worth vectors, which for convex games coincide with the usual marginal worth vectors, is the set of extreme points of the core of the assignment game.

On extreme points of the core and reduced games

Given a balanced cooperative game, we prove that the extreme points of the core have the reduced game property with respect to the Davis and Maschler reduced game. One particular case of this reduction gives when we name marginal games. These games allow us to define the reduced marginal worth vectors, where every player gets his marginal contribution to a successive marginal game. This set of vectors is proved to be the set of extreme points of the core of those balanced games which are almost convex, that is, those balanced games such that all proper subgames are convex.

A note on the nucleolus and the kernel of the assignment game

Abstract.There exist coalitional games with transferable utility which have the same core but different nucleoli. We show that this cannot happen in the case of assignment games. Whenever two assignment games have the same core, their nucleoli also coincide. To show this, we prove that the nucleolus of an assignment game coincides with that of its buyer–seller exact representative.

On the dimension of the core of the assignment game

The set of optimal matchings in the assignment matrix allows to define a reflexive and symmetric binary relation on each side of the market, the equal-partner binary relation. The number of equivalence classes of the transitive closure of the equal-partner binary relation determines the dimension of the core of the assignment game. This result provides an easy procedure to determine the dimension of the core directly from the entries of the assignment matrix and shows that the dimension of the core is not as much determined by the number of optimal matchings as by their relative position.

The Böhm–Bawerk horse market: a cooperative analysis

Single–valued solutions for the case of two-sided market games without product differentiation, also known as Böhm–Bawerk horse market games, are analyzed. The nucleolus is proved to coincide with the τ value, and is thus the midpoint of the core. The Shapley value is in the core only if the game is a square glove market, and in this case also coincides with the two aforementioned solutions.

A Simple Procedure to Obtain the Extreme Core Allocations of an Assignment Market

Given an assignment market, we introduce a set of vectors, one for each possible ordering on the player set, which we name the max-payoff vectors. Each one of these vectors is obtained recursively only making use of the assignment matrix. Those max-payoff vectors that are efficient turn out to give the extreme core allocations of the market. When the assignment game has a large core, all the max-payoff vectors are extreme core allocations.

Assignment markets with the same core

In the framework of bilateral assignment games, we study the set of matrices associated with assignment markets with the same core. We state conditions on matrix entries that ensure that the related assignment games have the same core. We prove that the set of matrices leading to the same core form a join-semilattice with a finite number of minimal elements and a unique maximum. We provide a characterization of the minimal elements. A sufficient condition under which the join-semilattice reduces to a lattice is also given.

An axiomatization of the nucleolus of assignment markets

On the domain of two-sided assignment markets with agents’ reservation values, the nucleolus is axiomatized as the unique solution that satisfies consistency with respect to Owen’s reduced game and symmetry of maximum complaints of the two sides. As an adjunt, we obtain a geometric characterization of the nucleolus by means of a strong form of the bisection property that characterizes the intersection between the core and the kernel of a coalitional game in (Math Opr Res 4:303–338, 1979).