Edge cover and polymatroid flow problems

In an n by n complete bipartite graph with independent exponentially distributed edge costs, we ask for the minimum total cost of a set of edges of which each vertex is incident to at least one. This so-called minimum edge cover problem is a relaxation of perfect matching. We show that the large n limit cost of the minimum edge cover is W (1)2 + 2W (1) ≈ 1.456, where W is the Lambert W -function. In particular this means that the minimum edge cover is essentially cheaper than the minimum perfect matching, whose limit cost is π2/6≈ 1.645. We obtain this result through a generalization of the perfect matching problem to a setting where we impose a (poly-)matroid structure on the two vertex-sets of the graph, and ask for an edge set of prescribed size connecting independent sets.