Rainbow connection number, bridges and radius

TL;DR

This paper investigates the rainbow connection number of connected graphs, providing bounds based on radius and bridges, and generalizes previous results by Basavaraju et al. with new structural insights.

Contribution

The paper introduces a method to bound the rainbow connection number using connected dominating sets and bridges, extending prior bounds to more general graph classes.

Findings

Bound on $rc(G)$ in terms of radius and bridges.

Construction of connected dominating sets with controlled rainbow connection.

Generalization of Basavaraju et al.'s result to broader graph classes.

Abstract

Let $G$ be a connected graph. The notion \emph{the rainbow connection number $rc(G)$} of a graph $G$ was introduced recently by Chartrand et al. Basavaraju et al. showed that for every bridgeless graph $G$ with radius $r$, $rc(G)\leq r(r+2)$, and the bound is tight. In this paper, we prove that if $G$ is a connected graph, and $D^{k}$ is a connected $k$-step dominating set of $G$, then $G$ has a connected $(k-1)$-step dominating set $D^{k-1}\supset D^{k}$ such that $rc(G[D^{k-1}])\leq rc(G[D^{k}])+\max\{2k+1,b_k\}$, where $b_k$ is the number of bridges in $ E(D^{k}, N(D^{k}))$. Furthermore, for a connected graph $G$ with radius $r$, let $u$ be the center of $G$, and $D^{r}=\{u\}$. Then $G$ has $r-1$ connected dominating sets $ D^{r-1}, D^{r-2},..., D^{1}$ satisfying $D^{r}\subset D^{r-1}\subset D^{r-2} ...\subset D^{1}\subset D^{0}=V(G)$, and $rc(G)\leq \sum_{i=1}^{r}\max\{2i+1,b_i\}$, where $b_i$ is the number of bridges in $ E(D^{i}, N(D^{i})), 1\leq i \leq r$. From the result, we can get that if for all $1\leq i\leq r, b_i\leq 2i+1$, then $rc(G)\leq \sum_{i=1}^{r}(2i+1)= r(r+2)$; if for all $1\leq i\leq r, b_i> 2i+1$, then $rc(G)= \sum_{i=1}^{r}b_i$, the number of bridges of $G$. This generalizes the result of Basavaraju et al.