Upper bound on the number of edges of an almost planar bipartite graph

Abstract

Let $G$ be a bipartite graph without loops and multiple edges on $v\ge 4$ vertices, which can be drawn on the plane such that any edge intersects at most one other edge. We prove that such graph has at most $3v-8$ edges for even $v\ne 6$ and at most $3v-9$ edges for odd $v$ and $v=6$. For all $v\ge 4$ examples showing that these bounds are tight are constructed. In the end of paper we discuss a question about drawings of complete bipartite graphs on the plane such that any edge intersects at most one other edge. {\sc Keywords:} topological graphs, planar graphs, bipartite graphs.