Polytopes, permanents and graphs with large factors

Randomized algorithms for approximating the number of perfect matchings in a graph are considered. An algorithm that is a natural simplification of one suggested and analyzed previously is introduced and analyzed. One of the key ideas is to view the analysis from a geometric perspective: it is proved that for any graph G the k-slice of the well-known Edmonds matching polytope has magnification 1. For a bipartite graph G=(U, V, E), |U|=|V|=n, with d edge-disjoint perfect matchings, it is proved that the ratio of the number of almost perfect matchings to the number of perfect matchings is at most n³ⁿd/. For any constant α>0 this yields a a fully polynomial randomized algorithm for approximating the number of perfect matchings in bipartite graphs with d⩾αn. Moreover, for some constant c>0 it is the fastest known approximation algorithm for bipartite graphs with d⩾ clog n