Simultaneous matchings: Hardness and approximation

摘要

Given a bipartite graph G=(X∪˙D,E⊆X×D), an X-perfect matching is a matching in G that covers every node in X. In this paper we study the following generalisation of the X-perfect matching problem, which has applications in constraint programming: Given a bipartite graph as above and a collection F⊆2X of k subsets of X, find a subset M⊆E of the edges such that for each C∈F, the edge set M∩(C×D) is a C-perfect matching in G (or report that no such set exists). We show that the decision problem is NP-complete and that the corresponding optimisation problem is in APX when k=O(1) and even APX-complete already for k=2. On the positive side, we show that a 2/(k+1)-approximation can be found in poly(k,|X∪D|) time. We show also that such an approximation M can be found in time (k+(k2)2k−2)poly(|X∪D|), with the further restriction that each vertex in D has degree at most 2 in M.