论文标题

平面图的灵活性 - 锐化工具以获取四个尺寸的列表

Flexibility of Planar Graphs -- Sharpening the Tools to Get Lists of Size Four

论文作者

Choi, Ilkyoo, Clemen, Felix Christian, Ferrara, Michael, Horn, Paul, Ma, Fuhong, Masařík, Tomáš

论文摘要

每个顶点$ v $具有可用颜色的列表$ l(v)$的图形,如果有适当的颜色,则每种$ v $的颜色为$ v $的颜色,则可以$ l $ - 色。如果每个任务$ l $至少$ k $颜色的每个顶点都可以保证$ l $颜色,则图形为$ k $ -Choosable。给定列表分配$ l $,一个顶点$ v $的$ l $ - 重复是l(v)$中的颜色$ c \。在本文中,我们研究了[Z. Dvočhk,S。Norin和L. Postle:带有请求的列表着色。 J. Graph Doemhy 2019],其中必须满足所请求的一组预状的“足够”,而不是所有人。图$ g $是$ \ varepsilon $ - 列表尺寸$ k $的flexible,如果任何$ k $ list tissmentment $ l $,以及任何$ l $ l $ requests的$ s $ s $ s $ s $ s $ l $ l $ coloring $ l $ g $的$ g $满足$ \ varepsilon $ fraction $ f varepsilon $ fraction $ $ s $ $ s $。据推测,平面图是$ \ varepsilon $ - 列表尺寸$ 5 $的fleflex,但仅适用于列表尺寸$ 6 $和某些平面图的子类。我们给出了上述结果证明中使用的主要工具的更强版本。通过这样做,我们改进了Masa树的结果,并表明没有$ k_4^ - $的平面图是$ \ varepsilon $ - 列表尺寸$ 5 $。我们还证明,没有$ 4 $ CYCLES和$ 3 $ CYCLE距离的平面图至少2 $ \ Varepsilon $ - 列表尺寸$ 4 $。最后,我们介绍了$ \ varepsilon $ - flexibility的一种新形式(稍弱),每个顶点都有一个请求。在这种情况下,我们提供了一个更强大的工具,并证明了它的有用性,可以进一步扩展$ \ varepsilon $的图形类别的列表尺寸$ 5 $。

A graph where each vertex $v$ has a list $L(v)$ of available colors is $L$-colorable if there is a proper coloring such that the color of $v$ is in $L(v)$ for each $v$. A graph is $k$-choosable if every assignment $L$ of at least $k$ colors to each vertex guarantees an $L$-coloring. Given a list assignment $L$, an $L$-request for a vertex $v$ is a color $c\in L(v)$. In this paper, we look at a variant of the widely studied class of precoloring extension problems from [Z. Dvořák, S. Norin, and L. Postle: List coloring with requests. J. Graph Theory 2019], wherein one must satisfy "enough", as opposed to all, of the requested set of precolors. A graph $G$ is $\varepsilon$-flexible for list size $k$ if for any $k$-list assignment $L$, and any set $S$ of $L$-requests, there is an $L$-coloring of $G$ satisfying an $\varepsilon$-fraction of the requests in $S$. It is conjectured that planar graphs are $\varepsilon$-flexible for list size $5$, yet it is proved only for list size $6$ and for certain subclasses of planar graphs. We give a stronger version of the main tool used in the proofs of the aforementioned results. By doing so, we improve upon a result by Masařík and show that planar graphs without $K_4^-$ are $\varepsilon$-flexible for list size $5$. We also prove that planar graphs without $4$-cycles and $3$-cycle distance at least 2 are $\varepsilon$-flexible for list size $4$. Finally, we introduce a new (slightly weaker) form of $\varepsilon$-flexibility where each vertex has exactly one request. In that setting, we provide a stronger tool and we demonstrate its usefulness to further extend the class of graphs that are $\varepsilon$-flexible for list size $5$.

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