Graphs With Large Semipaired Domination Number

Let $G$ be a graph with vertex set $V$ and no isolated vertices. A subset $ S \subseteq V $ is a semipaired dominating set of $G$ if every vertex in $ V \backslash S $ is adjacent to a vertex in $S$ and $S$ can be partitioned into two element subsets such that the vertices in each subset are at most distance two apart. The semipaired domination number $ \gamma_{pr2}(G) $ is the minimum cardinality of a semipaired dominating set of $G$. We show that if $G$ is a connected graph $G$ of order $ n \ge 3 $, then \( \gamma_{pr2} (G) \le \tfrac{2}{3} n \), and we characterize the extremal graphs achieving equality in the bound.