Vertex covers and secure domination in graphs

Let G = (V, E) be a graph and let S ⊆ V. The set S is a dominating set of G if every vertex in V \ S is adjacent to some vertex in S. The set S is a secure dominating set of G if for each u ∈V \ S, there exists a vertex v ∈ S such that uv ∈ E and (S \ {v}) ∪ {u}is a dominating set of G. The minimum cardinality of a secure dominating set in G is the secure domination number γ_s(G) of G. We show that if G is a connected graph of order n with minimum degree at least two that is not a 5-cycle, then γ_s (G) ≤ n/2 and this bound is sharp. Our proof uses a covering of a subset of V(G) by vertex-disjoint copies of subgraphs each of which is isomorphic to K₂ or to an odd cycle.