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Keywords:
traceable graph; line graph; spanning trail; closure
traceable graph; line graph; spanning trail; closure
Summary:
The line graph of a graph $G$, denoted by $L(G)$, has $E(G)$ as its vertex set, where two vertices in $L(G)$ are adjacent if and only if the corresponding edges in $G$ have a vertex in common. For a graph $H$, define $\bar {\sigma }_2 (H) = \min \{ d(u) + d(v) \colon uv \in E(H)\}$. Let $H$ be a 2-connected claw-free simple graph of order $n$ with $\delta (H)\geq 3$. We show that, if $\bar {\sigma }_2 (H) \geq \frac 17 (2n -5)$ and $n$ is sufficiently large, then either $H$ is traceable or the Ryjáček's closure ${\rm cl}(H)=L(G)$, where $G$ is an essentially $2$-edge-connected triangle-free graph that can be contracted to one of the two graphs of order 10 which have no spanning trail. Furthermore, if $\bar {\sigma }_2 (H) > \frac 13 (n-6)$ and $n$ is sufficiently large, then $H$ is traceable. The bound $\frac 13 (n-6)$ is sharp. As a byproduct, we prove that there are exactly eight graphs in the family ${\mathcal G}$ of 2-edge-connected simple graphs of order at most 11 that have no spanning trail, an improvement of the result in Z. Niu et al. (2012).
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