Total Domination in Graphs with Diameter 2



The total domination number math formula of a graph G is the minimum cardinality of a set S of vertices, so that every vertex of G is adjacent to a vertex in S. In this article, we determine an optimal upper bound on the total domination number of a graph with diameter 2. We show that for every graph G on n vertices with diameter 2, math formula. This bound is optimal in the sense that given any math formula, there exist graphs G with diameter 2 of all sufficiently large even orders n such that math formula.