Average eccentricity, minimum degree and maximum degree in graphs

Abstract

Let $G$ be a connected finite graph with vertex set $V(G)$. The eccentricity $e(v)$ of a vertex $v$ is the distance from $v$ to a vertex farthest from $v$. The average eccentricity of $G$ is defined as $\frac{1}{|V(G)|}\sum_{v \in V(G)}e(v)$. We show that the average eccentricity of a connected graph of order $n$, minimum degree $\delta$ and maximum degree $\Delta$ does not exceed $\frac{9}{4} \frac{n-\Delta-1}{\delta+1} \big( 1 + \frac{\Delta-\delta}{3n} \big) + 7$, and this bound is sharp apart from an additive constant. We give improved bounds for triangle-free graphs and for graphs not containing a $4$-cycles.