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arXiv 2309.04450math.CO
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$3$-临界符号图的密度

Density of $3$-critical signed graphs

Laurent Beaudou, Penny Haxell, Kathryn Nurse, Sagnik Sen, Zhouningxin Wang

AI总结:

本文扩展了圆形可着色性定义,证明了 $n$ 个顶点的 $3$-临界符号图至少有 $\ rac{3n-1}{2}$ 条边且界限渐近紧确,并得出围长至少为 $6$ 的符号平面图和射影平面图的相关 $3$-可着色性结论。

摘要:

如果一个符号图不是 $k$-可着色的,但它的每一个真子图都是 $k$-可着色的,我们就称该符号图为 $k$-临界的。使用 Naserasr、Wang 和 Zhu 提出的可着色性定义(该定义扩展了圆形可着色性的概念),我们证明了在 $n$ 个顶点上的每个 $3$-临界符号图至少有 $\ rac{3n-1}{2}$ 条边,并且该界限是渐近紧确的。由此可得,围长至少为 $6$ 的每个符号平面图或射影平面图都是(圆形)$3$-可着色的,并且对于射影平面的情况,该围长条件是最佳的。为了证明我们的主要结果,我们将其重新表述为关于到符号图 $C_{3}^*$ 的同态存在性的问题,其中 $C_{3}^*$ 是在每个顶点上增加一个负自环的正三角形。

英文摘要:

We say that a signed graph is $k$-critical if it is not $k$-colorable but every one of its proper subgraphs is $k$-colorable. Using the definition of colorability due to Naserasr, Wang, and Zhu that extends the notion of circular colorability, we prove that every $3$-critical signed graph on $n$ vertices has at least $\frac{3n-1}{2}$ edges, and that this bound is asymptotically tight. It follows that every signed planar or projective-planar graph of girth at least $6$ is (circular) $3$-colorable, and for the projective-planar case, this girth condition is best possible. To prove our main result, we reformulate it in terms of the existence of a homomorphism to the signed graph $C_{3}^*$, which is the positive triangle augmented with a negative loop on each vertex.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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