Graphs with Large Hop Roman Domination Number

摘要

A subset $S$ of vertices of a graph $G$ is a hop dominating set if every vertex outside $S$ is at distance two from a vertex of $S$. A Roman dominating function on a graph $G=(V,E)$ is a function $f: V(G) longrightarrow {0, 1, 2}$ satisfying the condition that every vertex $u$ for which $f(u)=0$ is adjacent to at least one vertex $v$ for which $f(v)=2$. A hop Roman dominating function (HRDF) of $G$ is a function $f: V(G) longrightarrow {0, 1, 2}$ having the property that for every vertex $v in V$ with $f(v) = 0$ there is a vertex $u$ with $f(u)=2$ and $d(u,v)=2$. The weight of a HRDF $f$ is the sum $f (V) = sum_{vin V} f(v)$. The minimum weight of a HRDF on $G$ is called the hop Roman domination number of $G$ and is denoted by $gamma_{hR}(G)$. In this paper we characterize all graphs $G$ of order $n$ with $gamma_{hR}(G)=n$ or $gamma_{hR}(G)=n-1$.