Matroid intersection

In combinatorial optimization, the matroid intersection problem is to find a largest common independent set in two matroids over the same ground set. If the elements of the matroid are assigned real weights, the weighted matroid intersection problem is to find a common independent set with the maximum possible weight. These problems generalize many problems in combinatorial optimization including finding maximum matchings and maximum weight matchings in bipartite graphs and finding arborescences in directed graphs.

The matroid intersection theorem, due to Jack Edmonds, says that there is always a simple upper bound certificate, consisting of a partitioning of the ground set amongst the two matroids, whose value (sum of respective ranks) equals the size of a maximum common independent set. Based on this theorem, the matroid intersection problem for two matroids can be solved in polynomial time using matroid partitioning algorithms.

Let G = (U,V,E) be a bipartite graph. One may define a partition matroid M_U on the ground set E, in which a set of edges is independent if no two of the edges have the same endpoint in U. Similarly one may define a matroid M_V in which a set of edges is independent if no two of the edges have the same endpoint in V. Any set of edges that is independent in both M_U and M_V has the property that no two of its edges share an endpoint; that is, it is a matching. Thus, the largest common independent set of M_U and M_V is a maximum matching in G.

The matroid intersection problem becomes NP-hard when three matroids are involved, instead of only two.

...
Wikipedia