论文标题

polyhedra I中的循环的动力学:隔离引理

Dynamics of Cycles in Polyhedra I: The Isolation Lemma

论文作者

Kessler, Jan, Schmidt, Jens M.

论文摘要

如果$ g-v(c)$的每个组件都是一个顶点,则图$ g $的周期$ c $是\ emph {隔离}。我们表明,在多面体图中的隔离周期可以扩展到较大的周期:每个隔离周期$ c $ a length $ 6 \ leq | e(c)| <\ left \ lfloor \ frac {2} {3}(| v(g)| +4)\ right \ rfloor $意味着较大长度的隔离周期$ c'$,其中包含$ v(c)$。通过“迭代”到如此较大的周期中,我们获得了一个强大且非常通用的电感电动机,以证明长周期并计算它们(我们将提供具有二次运行时间的算法)。这是迄今为止难以捉摸的寻求通用感应的第一步,该通用感应捕获了最长的多面体图类别的循环。 我们的电动机还提供了一种证明在Tutte循环长度上线性下限的方法,因为如果$ c $是,$ c'$将是$ g $的tutte循环。我们还证明了$ | e(c')| \ leq | e(c)|+3 $如果$ g $不包含第五尺寸的面孔,从而为循环光谱提供了新的工具,并提供了证据表明,五个面孔的面孔可能会阻碍许多图类别的长期循环。我们在以下猜想上测试了有关基本4连接图的猜想。 如果平面图是\ emph {基本上是$ 4 $ - 连接},则它是3键连接的,并且其3分隔符中的每一个都是单个顶点的邻域。杰克逊(Jackson)和沃尔马德(Wormald)证明,$ n $ dertices上的每一个基本4连接的平面图$ g $都包含一个长度至少$ \ frac {2} {2} {5} {5}(n+2)$,并且最近多次改善了该结果,较低的$ \ frac $ \ frac {5} {5} {5} {8} {n+2)(n+2)。但是,当前最著名的上限是由无限的图形族给出的,其中没有图$ g $包含一个比$ \ weft \ lfloor \ frac {2} {3} {3} {3}(n+4)\ right \ rfloor $更长的周期;该上限仍然是无与伦比的。 使用隔离循环,我们改善了下限以匹配上部。我们所有的结果都很紧。

A cycle $C$ of a graph $G$ is \emph{isolating} if every component of $G-V(C)$ is a single vertex. We show that isolating cycles in polyhedral graphs can be extended to larger ones: every isolating cycle $C$ of length $6 \leq |E(C)| < \left \lfloor \frac{2}{3}(|V(G)|+4) \right \rfloor$ implies an isolating cycle $C'$ of larger length that contains $V(C)$. By "hopping" iteratively to such larger cycles, we obtain a powerful and very general inductive motor for proving long cycles and computing them (we will give an algorithm with quadratic running time). This is the first step towards the so far elusive quest of finding a universal induction that captures longest cycles of polyhedral graph classes. Our motor provides also a method to prove linear lower bounds on the length of Tutte cycles, as $C'$ will be a Tutte cycle of $G$ if $C$ is. We prove in addition that $|E(C')| \leq |E(C)|+3$ if $G$ contains no face of size five, which gives a new tool for results about cycle spectra, and provides evidence that faces of size five may obstruct long cycles in many graph classes. We test our motor on the following conjecture about essentially 4-connected graphs. A planar graph is \emph{essentially $4$-connected} if it is 3-connected and every of its 3-separators is the neighborhood of a single vertex. Jackson and Wormald proved that every essentially 4-connected planar graph $G$ on $n$ vertices contains a cycle of length at least $\frac{2}{5}(n+2)$, and this result has recently been improved multiple times, culminating in the lower bound $\frac{5}{8}(n+2)$. However, the currently best known upper bound is given by an infinite family of such graphs in which no graph $G$ contains a cycle that is longer than $\left \lfloor \frac{2}{3}(n+4) \right \rfloor$; this upper bound is still unmatched. Using isolating cycles, we improve the lower bound to match the upper. All our results are tight.

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