Dominating Vertex Covers: The Vertex-Edge Domination Problem

The vertex-edge domination number of a graph, γ_ve(G), is defined to be the cardinality of a smallest set D such that there exists a vertex cover C of G such that each vertex in C is dominated by a vertex in D. This is motivated by the problem of determining how many guards are needed in a graph so that a searchlight can be shone down each edge by a guard either incident to that edge or at most distance one from a vertex incident to the edge. Our main result is that for any cubic graph G with n vertices, γ_ve(G) ≤ 9n/26. We also show that it is NP-hard to decide if γ_ve(G) = γ(G) for bipartite graph G.