Towards the Overfull Conjecture

Abstract

Let $G$ be a simple graph with maximum degree denoted as $\Delta(G)$. An overfull subgraph $H$ of $G$ is a subgraph satisfying the condition $|E(H)| > \Delta(G)\lfloor \frac{1}{2}|V(H)| \rfloor$. In 1986, Chetwynd and Hilton proposed the Overfull Conjecture, stating that a graph $G$ with maximum degree $\Delta(G)> \frac{1}{3}|V(G)|$ has chromatic index equal to $\Delta(G)$ if and only if it does not contain any overfull subgraph. The Overfull Conjecture has many implications. For example, it implies a polynomial-time algorithm for determining the chromatic index of graphs $G$ with $\Delta(G) > \frac{1}{3}|V(G)|$, and implies several longstanding conjectures in the area of graph edge colorings. In this paper, we make the first breakthrough towards the conjecture when not imposing a minimum degree condition on the graph: for any $0<\varepsilon \le \frac{1}{14}$, there exists a positive integer $n_0$ such that if $G$ is a graph on $n\ge n_0$ vertices with $\Delta(G) \ge (1-\varepsilon)n$, then the Overfull Conjecture holds for $G$. The previous best result in this direction, due to Chetwynd and Hilton from 1989, asserts the conjecture for graphs $G$ with $\Delta(G) \ge |V(G)|-3$. Our result also implies the Average Degree Conjecture of Vizing from 1968 for the same class of graphs $G$.