Determining crossing numbers of the join products of two specific graphs of order six with the discrete graph

Abstract

The main aim of the paper is to give the crossing number of the join product G* + Dn for the connected graph G* of order six consisting of P4 + D1 and of one leaf incident with some inner vertex of the path P4 on four vertices, and where Dn consists of n isolated vertices. In the proofs, it will be extend the idea of the minimum numbers of crossings between two different subgraphs from the set of subgraphs which do not cross the edges of the graph G* onto the set of subgraphs by which the edges of G* are crossed exactly once. Due to the mentioned algebraic topological approach, we are able to extend known results concerning crossing numbers for join products of new graphs. The proofs are done with the help of software that generates all cyclic permutations for a given number k, and creates a new graph COG for calculating the distances between all (k-1)! vertices of the graph. Finally, by adding one edge to the graph G*, we are able to obtain the crossing number of the join product of one other graph with the discrete graph Dn.

On the crossing numbers of the join products of five graphs on six vertices with discrete graph

“…The exact values for the crossing numbers of G + D n for all graphs G of order at most four are given by Klešč and Schr ötter [21]. Also, the crossing numbers of the graphs G + D n are known for a lot of graphs G of order five and six [1,5,7,10,11,12,13,15,17,18,19,20,22,23,26,27,29,30,33,34,35,36]. In all these cases, the graph G is connected and contains usually at least one cycle.…”