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arXiv 2107.06111cs.CCcs.DS
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参数化复杂性的精确结构阈值

Towards exact structural thresholds for parameterized complexity

Falko Hegerfeld, Stefan Kratsch

摘要:

参数化复杂性试图使用输入结构来获得NP难问题的更快算法。这对于低树宽的图是最成功的:许多问题允许相对于树宽的快速算法,其中许多在SETH下是最优的。更一般的结构,如低candle-width和更严格的结构,如低删除距离稀疏图类,这样的结果是已知的。尽管取得了这些成功,但这些成果仍然是可能结构范围内的“孤岛”。而不是添加更多的岛屿,我们试图确定它们之间的过渡,也就是说,我们的目标是结构阈值的复杂性增加输入结构变得更加普遍。从删除距离到树宽,是一个单一的删除集的图与简单的组件足以产生相同的下界作为树宽或它需要许多不相交的分隔符?从树宽到宽,密度增加多少才能带来与宽相同的复杂性?相反,什么是最严格的结构,产生相同的下限?对于树宽,我们得到改进的和新的下界,已经适用于图形与一个单一的分隔符$X$,使$G-X$具有树宽$r=O(1)$,而$G$具有树宽$|X|+O(1)$。我们排除了在时间$O^*((r+1-ε)^{k})$中运行的算法,用于删除$r$ -由$k=|X|$参数化的可着色。对于candle-width,我们排除了删除到$r$ -Colorable的时间$O^*((2^r-ε)^k)$,其中$X$现在允许由$k$双类组成。在顶点覆盖、支配集和最大割上有进一步的结果。所有的下限匹配现有的和新设计的算法。

英文摘要:

Parameterized complexity seeks to use input structure to obtain faster algorithms for NP-hard problems. This has been most successful for graphs of low treewidth: Many problems admit fast algorithms relative to treewidth and many of them are optimal under SETH. Fewer such results are known for more general structure such as low clique-width and more restrictive structure such as low deletion distance to a sparse graph class. Despite these successes, such results remain "islands'' within the realm of possible structure. Rather than adding more islands, we seek to determine the transitions between them, that is, we aim for structural thresholds where the complexity increases as input structure becomes more general. Going from deletion distance to treewidth, is a single deletion set to a graph with simple components enough to yield the same lower bound as for treewidth or does it take many disjoint separators? Going from treewidth to clique-width, how much more density entails the same complexity as clique-width? Conversely, what is the most restrictive structure that yields the same lower bound? For treewidth, we obtain both refined and new lower bounds that apply already to graphs with a single separator $X$ such that $G-X$ has treewidth $r=O(1)$, while $G$ has treewidth $|X|+O(1)$. We rule out algorithms running in time $O^*((r+1-ε)^{k})$ for Deletion to $r$-Colorable parameterized by $k=|X|$. For clique-width, we rule out time $O^*((2^r-ε)^k)$ for Deletion to $r$-Colorable, where $X$ is now allowed to consist of $k$ twinclasses. There are further results on Vertex Cover, Dominating Set and Maximum Cut. All lower bounds are matched by existing and newly designed algorithms.

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