On Domination Number and Distance in Graphs

TL;DR

This paper establishes a precise relationship between the domination number and distances among vertices in graphs, revealing new bounds and confirming a conjecture related to eccentricity and boundary vertices.

Contribution

It determines the exact value of the constant $C_r$ for all $r \,\geq\, 3$, linking domination number to average distances and Wiener index in graphs.

Findings

$C_r=\frac{1}{r(r-1)}$ for all $r\geq 3$

Established the bound $\gamma(G) \geq \frac{1}{n(n-1)}W(G)$

Proved a conjecture relating domination number and eccentricity of boundary vertices.

Abstract

A vertex set $S$ of a graph $G$ is a \emph{dominating set} if each vertex of $G$ either belongs to $S$ or is adjacent to a vertex in $S$. The \emph{domination number} $\gamma(G)$ of $G$ is the minimum cardinality of $S$ as $S$ varies over all dominating sets of $G$. It is known that $\gamma(G) \ge \frac{1}{3}(diam(G)+1)$, where $diam(G)$ denotes the diameter of $G$. Define $C_r$ as the largest constant such that $\gamma(G) \ge C_r \sum_{1 \le i < j \le r}d(x_i, x_j)$ for any $r$ vertices of an arbitrary connected graph $G$; then $C_2=\frac{1}{3}$ in this view. The main result of this paper is that $C_r=\frac{1}{r(r-1)}$ for $r\geq 3$. It immediately follows that $\gamma(G)\geq \mu(G)=\frac{1}{n(n-1)}W(G)$, where $\mu(G)$ and $W(G)$ are respectively the average distance and the Wiener index of $G$ of order $n$. As an application of our main result, we prove a conjecture of DeLaVi\~{n}a et al.\;that $\gamma(G)\geq \frac{1}{2}(ecc_G(B)+1)$, where $ecc_G(B)$ denotes the eccentricity of the boundary of an arbitrary connected graph $G$.