Counting Maximum Matchings in Planar Graphs Is Hard

Abstract

Here we prove that counting maximum matchings in planar, bipartite graphs is #P-complete. This is somewhat surprising in the light that the number of perfect matchings in planar graphs can be computed in polynomial time. We also prove that counting non-necessarily perfect matchings in planar graphs is already #P-complete if the problem is restricted to bipartite graphs. So far hardness was proved only for general, non-necessarily bipartite graphs.

The presented result is not new. A proof of the main theorem appeared in Vadhan (2001) THE COMPLEXITY OF COUNTING IN SPARSE, REGULAR, AND PLANAR GRAPHS