Upper bounds for the achromatic and coloring numbers of a graph

Abstract

Dvo\v{r}\'ak \emph{et al.} introduced a variant of the Randi\'c index of a graph $G$, denoted by $R'(G)$, where $R'(G)=\sum_{uv\in E(G)}\frac 1 {\max\{d(u), d(v)\}}$, and $d(u)$ denotes the degree of a vertex $u$ in $G$. The coloring number $col(G)$ of a graph $G$ is the smallest number $k$ for which there exists a linear ordering of the vertices of $G$ such that each vertex is preceded by fewer than $k$ of its neighbors. It is well-known that $\chi(G)\leq col(G)$ for any graph $G$, where $\chi(G)$ denotes the chromatic number of $G$. In this note, we show that for any graph $G$ without isolated vertices, $col(G)\leq 2R'(G)$, with equality if and only if $G$ is obtained from identifying the center of a star with a vertex of a complete graph. This extends some known results. In addition, we present some new spectral bounds for the coloring and achromatic numbers of a graph.