Dichotomy Results for Classified Rank-Maximal Matchings and Popular Matchings
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In this paper, we consider the problem of computing an optimal matching in a bipartite graph where elements of one side of the bipartition specify preferences over the other side, and one or both sides can have capacities and classifications. The input instance is a bipartite graph G=(A U P,E), where A is a set of applicants, P is a set of posts, and each applicant ranks its neighbors in an order of preference, possibly involving ties. Moreover, each vertex v belonging to A U P has a quota q(v) denoting the maximum number of partners it can have in any allocation of applicants to posts -- referred to as a matching in this paper. A classification C_u for a vertex u is a collection of subsets of neighbors of u. Each subset (class) C∈C_u has an upper quota denoting the maximum number of vertices from C that can be matched to u. The goal is to find a matching that is optimal amongst all the feasible matchings, which are matchings that respect quotas of all the vertices and classes. We consider two well-studied notions of optimality namely popularity and rank-maximality. We present an O(|E|^2)-time algorithm for finding a feasible rank-maximal matching, when each classification is a laminar family. We complement this with an NP-hardness result when classes are non-laminar even under strict preference lists, and even when only posts have classifications, and each applicant has a quota of one. We show an analogous dichotomy result for computing a popular matching amongst feasible matchings (if one exists) where applicants having a quota of one. En-route to designing the polynomial time algorithms, we adapt the well-known Dulmage Mendelsohn decomposition of a bipartite graph w.r.t. a maximum matching to a maximum flow on special flow networks. We believe this generalization is of independent interest.
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