We consider the popular matching problem in a roommates instance with strict preference lists. While popular matchings always exist in a bipartite instance, they need not exist in a roommates instance. The complexity of the popular matching problem in a roommates instance has been an open problem for several years and here we show it is NP-hard. A sub-class of max-size popular matchings called dominant matchings has been well-studied in bipartite graphs. We show that the dominant matching problem in a roommates instance is also NP-hard and this is the case even when the instance admits a stable matching.
Techniques. We use properties of popular matchings in bipartite instances here --- in particular, we use the LP framework of popular matchings that was initiated in @cite_19 . Every popular matching @math in a bipartite instance @math is a max-weight perfect matching in a related graph @math and an optimal solution to the dual LP (dual to the max-weight perfect matching LP) is a witness to the popularity of @math . It is known that a matching in @math is popular if and only if it has a witness @math , where @math is the number of vertices.
@cite_19
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We study the problem of matching applicants to jobs under one-sided preferences; that is, each applicant ranks a non-empty subset of jobs under an order of preference, possibly involving ties. A matching M is said to be more popular than T if the applicants that prefer M to T outnumber those that prefer T to M. A matching is said to be popular if there is no matching more popular than it. Equivalently, a matching M is popular if @f(M,T)>[email protected](T,M) for all matchings T, where @f(X,Y) is the number of applicants that prefer X to Y. Previously studied solution concepts based on the popularity criterion are either not guaranteed to exist for every instance (e.g., popular matchings) or are NP-hard to compute (e.g., least unpopular matchings). This paper addresses this issue by considering mixed matchings. A mixed matching is simply a probability distribution over matchings in the input graph. The function @f that compares two matchings generalizes in a natural manner to mixed matchings by taking expectation. A mixed matching P is popular if @f(P,Q)>[email protected](Q,P) for all mixed matchings Q. We show that popular mixed matchings always exist and we design polynomial time algorithms for finding them. Then we study their efficiency and give tight bounds on the price of anarchy and price of stability of the popular matching problem.