Hamilton-connectivity of line graphs with application to their detour index

A graph is called Hamilton-connected if there exists a Hamiltonian path between every pair of its vertices. Determining whether or not a graph is Hamilton-connected is an NP-complete problem. In this paper, we present two methods to show Hamilton-connectivity in graphs. The first method uses the vertex connectivity and Hamiltoniancity of graphs, and, the second is the definition-based constructive method which constructs Hamiltonian paths between every pair of vertices. By employing these proof techniques, we show that the line graphs of the generalized Petersen, anti-prism and wheel graphs are Hamilton-connected. Combining it with some existing results, it shows that some of these families of Hamilton-connected line graphs have their underlying graph families non-Hamilton-connected. This, in turn, shows that the underlying graph of a Hamilton-connected line graph is not necessarily Hamilton-connected. As side results, the detour index being also an NP-complete problem, has been calculated for the families of Hamilton-connected line graphs. Finally, by computer we generate all the Hamilton-connected graphs on <= 7 vertices and all the Hamilton-laceable graphs on <= 10 vertices. Our research contributes towards our proposed conjecture that almost all graphs are Hamilton-connected.