Improved Enumeration of Simple Topological Graphs

Abstract

A simple topological graph T =(V(T), E(T)) is a drawing of a graph in the plane where every two edges have at most one common point (an endpoint or a crossing) and no three edges pass through a single crossing. Topological graphs G and H are isomorphic if H can be obtained from G by a homeomorphism of the sphere, and weakly isomorphic if G and H have the same set of pairs of crossing edges. We generalize results of Pach and Tóth and the author's previous results on counting different drawings of a graph under both notions of isomorphism. We prove that for every graph G with n vertices, m edges and no isolated vertices the number of weak isomorphism classes of simple topological graphs that realize G is at most 2O(n2 log(m/n)), and at most 2O(mn1/2log n) if m≤ n3/2. As a consequence we obtain a new upper bound 2O(n3/2 log n) on the number of intersection graphs of n pseudosegments. We improve the upper bound on the number of weak isomorphism classes of simple complete topological graphs with n vertices to 2n2 α (n)O(1), using an upper bound on the size of a set of permutations with bounded VC-dimension recently proved by Cibulka and the author. We show that the number of isomorphism classes of simple topological graphs that realize G is at most 2m2+O(mn) and at least 2Ω(m2) for graphs with m>(6+ε) n. © 2013 Springer Science+Business Media New York.