VOTING PATHS

We consider a variant of the popular matching problem here. The input instance is a bipartite graph G = (A boolean OR P, E), where vertices in A are called applicants and vertices in P are called posts. Each applicant ranks a subset of posts in an order of preference, possibly involving ties. A matching M is popular if there is no other matching M' such that the number of applicants who prefer their partners in M' to M exceeds the number of applicants who prefer their partners in M to M'. However, the "more popular than" relation is not transitive; hence this relation is not a partial order, and thus there need not be a maximal element here. Indeed, there are simple instances that do not admit popular matchings. The questions of whether an input instance G admits a popular matching and how to compute one if it exists were studied earlier by Abraham et al. Here we study reachability questions among matchings in G, assuming that G = (A boolean OR P, E) admits a popular matching. A matching M-k is reachable from M-0 if there is a sequence of matchings < M-0, M-1, ... , M-k > such that each matching is more popular than its predecessor. Such a sequence is called a length-k voting path from M-0 to Mk. We show an interesting property of reachability among matchings in G: there is always a voting path of length at most 2 from any matching to some popular matching. Given a bipartite graph G = (A boolean OR P, E) with n vertices and m edges and any matching M-0 in G, we give an O(m root n) algorithm to compute a shortest-length voting path from M-0 to a popular matching; when preference lists are strictly ordered, we have an O(m + n) algorithm. This problem has applications in dynamic matching markets, where applicants and posts can enter and leave the market, and applicants can also change their preferences arbitrarily. After any change, the current matching may no longer be popular, in which case we are required to update it. However, our model demands that we switch from one matching to another only if there is consensus among the applicants to agree to the switch. Hence we need to update via a voting path that ends in a popular matching. Thus our algorithm has applications here.