Yu Yokoi

A Generalized Polymatroid Approach to Stable Matchings with Lower Quotas

Classified stable matching, proposed by Huang, describes a matching model between academic institutes and applicants, in which each institute has upper and lower quotas on classes, i.e., subsets of applicants. Huang showed that the problem to decide whether there exists a stable matching or not is NP-hard in general. On the other hand, he showed that the problem is solvable if classes form a laminar family. For this case, Fleiner and Kamiyama gave a concise interpretation in terms of matroids and showed the lattice structure of stable matchings.In this paper we introduce stable matchings on generalized matroids, extending the model of Fleiner and Kamiyama. We design a polynomial-time algorithm which finds a stable matching or reports the nonexistence. We also show that the set of stable matchings, if nonempty, forms a lattice with several significant properties. Furthermore, we extend this structural result to the polyhedral framework, which we call stable allocations on generalized polymatroids.

On the Lattice Structure of Stable Allocations in a Two-Sided Discrete-Concave Market

The stable allocation model is a many-to-many matching model in which each pair’s partnership is represented by a nonnegative integer. This paper establishes a link between two different formulations of this model: the choice function model studied thoroughly by Alkan and Gale and the discrete-concave (M♮-concave) value function model introduced by Eguchi, Fujishige, and Tamura. We show that the choice functions induced from M♮-concave value functions are endowed with consistency, persistence, and size monotonicity. This implies, by the result of Alkan and Gale, that the stable allocations for M♮-concave value functions form a distributive lattice with several significant properties such as polarity, complementarity, and uni-size property. Furthermore, we point out that these results can be extended for quasi M♮-concave value functions.

Envy-Free Matchings with Lower Quotas

While every instance of the Hospitals/Residents problem admits a stable matching, the problem with lower quotas (HR-LQ) has instances with no stable matching. For such an instance, we expect the existence of an envy-free matching, which is a relaxation of a stable matching preserving a kind of fairness property. In this paper, we investigate the existence of an envy-free matching in several settings, in which hospitals have lower quotas and not all doctor–hospital pairs are acceptable. We first provide an algorithm that decides whether a given HR-LQ instance has an envy-free matching or not. Then, we consider envy-freeness in the Classified Stable Matching model due to Huang (in: Procedings of 21st annual ACM-SIAM symposium on discrete algorithms (SODA2010), SIAM, Philadelphia, pp 1235–1253, 2010 ), i.e., each hospital has lower and upper quotas on subsets of doctors. We show that, for this model, deciding the existence of an envy-free matching is NP-hard in general, but solvable in polynomial time if quotas are paramodular.

List Supermodular Coloring with Shorter Lists

In 1995, Galvin proved that a bipartite graph G admits a list edge coloring if every edge is assigned a color list of length Δ(G) the maximum degree of the graph. This result was improved by Borodin, Kostochka and Woodall, who proved that G still admits a list edge coloring if every edge e=st is assigned a list of max{dG(s);dG(t)} colors. Recently, Iwata and Yokoi provided the list supermodular coloring theorem that extends Galvins result to the setting of Schrijvers supermodular coloring. This paper provides a common generalization of these two extensions of Galvins result.