AI中文摘要
设$G$为无环多重图,最大度为$\Delta(G)$,平均度为$\overline{d}(G)$,密度为$\Gamma(G)$,边色数为$\chi'(G)$。多重图$G$称为边-$\Delta$-临界的,如果$\Delta(G)=\Delta$,$\chi'(G)=\Delta(G)+1$,且对每个真子图$H\subset G$有$\chi'(H) \le \Delta(G)$。Vizing猜想:若$G$是$n$个顶点上的边-$\Delta$-临界简单图,则$\overline{d}(G) \ge \Delta-1+\tfrac{3}{n}$。受此启发,我们猜想每个边-$\Delta$-临界多重图$G$满足$\overline{d}(G) \ge \tfrac{2\Delta+2}{3}$,且该界是紧的。我们首先给出该方向上的一个一般下界。对任意这样的图$G$,\\[ \overline{d}(G) \ge \begin{cases} \frac{\sqrt{17}-3}{2}(\Delta+1) & \text{if } \Delta \le 112;\\\\[4pt] \frac{\Delta+\sqrt{2\Delta-1}}{2} & \text{if } \Delta \ge 113. \end{cases} \\] 在重数$\mu$的附加条件下,该界可进一步改进。此时,\\[ \overline{d}(G)\ge \min\left\{ \frac{2\mu\Delta+2\mu(2\mu-1)}{4\mu-1},\\; \frac{\sqrt{17}-3}{2}(\Delta+1) \right\}. \\] 我们还证实了该猜想对$\Delta \in \{2,3,4,5,6,7,8\}$成立。作为推论,Goldberg猜想~\cite{Goldberg1984}对$\Delta(G)\in\{2,3,4,5\}$成立,即每个满足$\chi'(G)\ge \Delta(G)+1$的多重图$G$有$\Gamma(G)\ge \Delta(G)$。
英文摘要
Let $G$ be a loopless multigraph with maximum degree $Δ(G)$, average degree $\overline{d}(G)$, density $Γ(G)$, and chromatic index $χ'(G)$. A multigraph $G$ is called edge-$Δ$-critical if $Δ(G)=Δ$, $χ'(G)=Δ(G)+1$ and $χ'(H) \le Δ(G)$ for every proper subgraph $H\subset G$. Vizing conjectured that if $G$ is an edge-$Δ$-critical simple graph on $n$ vertices, then $\overline{d}(G) \ge Δ-1+\tfrac{3}{n}$. Motivated by this, we conjecture that every edge-$Δ$-critical multigraph $G$ satisfies $\overline{d}(G) \ge \tfrac{2Δ+2}{3}$, which is best possible. We first give a general lower bound in this direction. For any such graph $G$, \[ \overline{d}(G) \ge \begin{cases} \frac{\sqrt{17}-3}{2}(Δ+1) & \text{if } Δ\le 112;\\[4pt] \frac{Δ+\sqrt{2Δ-1}}{2} & \text{if } Δ\ge 113. \end{cases} \] This bound can be further improved under an additional condition on the multiplicity $μ$. In this case, \[ \overline{d}(G)\ge \min\left\{ \frac{2μΔ+2μ(2μ-1)}{4μ-1},\; \frac{\sqrt{17}-3}{2}(Δ+1) \right\}. \] We also confirm the conjecture for $Δ\in \{2,3,4,5,6,7,8\}$. As a consequence, Goldberg's conjecture~\cite{Goldberg1984} holds for $Δ(G)\in\{2,3,4,5\}$, that is, every multigraph $G$ with $χ'(G)\ge Δ(G)+1$ satisfies $Γ(G)\ge Δ(G)$.