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Algorithmica

, Volume 74, Issue 1, pp 440–465 | Cite as

The Hospitals/Residents Problem with Lower Quotas

  • Koki Hamada
  • Kazuo Iwama
  • Shuichi MiyazakiEmail author
Article

Abstract

The Hospitals/Residents problem is a many-to-one extension of the stable marriage problem. In an instance, each hospital specifies a quota, i.e., an upper bound on the number of positions it provides. It is well-known that in any instance, there exists at least one stable matching, and finding one can be done in polynomial time. In this paper, we consider an extension in which each hospital specifies not only an upper bound but also a lower bound on its number of positions. In this setting, there can be instances that admit no stable matching, but the problem of asking if there is a stable matching is solvable in polynomial time. In case there is no stable matching, we consider the problem of finding a matching that is “as stable as possible”, namely, a matching with a minimum number of blocking pairs. We show that this problem is hard to approximate within the ratio of \((|H|+|R|)^{1-\epsilon }\) for any positive constant \(\epsilon \) where \(H\) and \(R\) are the sets of hospitals and residents, respectively. We then tackle this hardness from two different angles. First, we give an exponential-time exact algorithm whose running time is \(O((|H||R|)^{t+1})\), where \(t\) is the number of blocking pairs in an optimal solution. Second, we consider another measure for optimization criteria, i.e., the number of residents who are involved in blocking pairs. We show that this problem is still NP-hard but has a polynomial-time \(\sqrt{|R|}\)-approximation algorithm.

Keywords

The stable marriage problem The Hospitals/Residents problem  Stable matching Approximation algorithm 

Notes

Acknowledgments

The authors would like to thank David F. Manlove for his valuable comments on this work. We also would like to thank anonymous reviewers for their helpful comments.

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.NTT Secure Platform Laboratories, NTT CorporationMusashino-shiJapan
  2. 2.Graduate School of InformaticsKyoto UniversityKyotoJapan
  3. 3.Academic Center for Computing and Media StudiesKyoto UniversityKyotoJapan

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