Finding Witnesses for Stability in the Hospitals/Residents Problem

The Hospitals/Residents problem is a many-to-one generalization of the well-known Stable Marriage problem. Its instance consists of a set of residents, a set of hospitals, each resident's preference list, each hospital's preference list, and each hospital's capacity (i.e., the number of available positions). It asks to find a stable matching between residents and hospitals. In this paper, we consider the problem of deciding, given residents' preference lists and a matching, whether there are hospitals' preference lists that make a given matching stable. We call this problem Stable Hospital's Preference List problem (SHPL). It is easy to see that there always exists a solution if we allow arbitrary preference lists of hospitals. Considering more suitable situations, we pose a restricted version, called k-SHPL, in which there are only k kinds of preference lists of hospitals. We show that 1-SHPL is solvable in polynomial time, while k-SHPL is NP-complete for any k such that 2 ≤ k ≤ n1-ε, where n is the number of residents and ε is any positive constant. We also present four heuristics algorithms (first-fit algorithms) for 2-SHPL. We implement these algorithms and present a computational study using random instances