arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1708.07591cs.DS
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树模式子图同构的条件下界

Conditional Lower Bound for Subgraph Isomorphism with a Tree Pattern

Robert Krauthgamer, Ohad Trabelsi

摘要:

kTree问题是子图同构的一种特殊情况,其中模式图是一棵树,也就是说,输入是一个$n$ -节点图$G$和一个$k$ -节点树$T$,目标是确定$G$是否有一个与$T$同构的子图.我们提供的证据表明,该问题的计算速度不能明显快于$2^{k} \textsf{poly}(n)$,这与Koutis和Williams已知的该问题的最快算法相匹配[ICALP 2009和TALG 2016]。具体来说,我们证明了如果kTree可以在时间$(2-\varepsilon)^k \textsf{poly}(n)$中求解某些常数$\varepsilon>0$,那么具有$n'$元素和$m'$集合的集合覆盖可以在时间$(2-δ)^{n'} \textsf{poly}(m')$中求解常数$δ(\varepsilon) > 0$,这将反驳Cygan等人的集合覆盖猜想。[CCC 2012和TALG 2016]。我们的技术产生了一个新的算法的p-部分覆盖问题,一个参数化版本的集覆盖,需要覆盖至少$p$元素(而不是所有元素)。对于任何固定的$\varepsilon>0$,其运行时间为$(2+\varepsilon)^p (m')^{O(1/\varepsilon)}$,这改进了Zehavi [ESA 2015]之前的$2.597^p \textsf{poly}(m')$时间算法。我们的运行时间几乎是最优的,因为$(2-\varepsilon')^p \textsf{poly}(m')$时间算法会反驳集合覆盖猜想。

英文摘要:

The kTree problem is a special case of Subgraph Isomorphism where the pattern graph is a tree, that is, the input is an $n$-node graph $G$ and a $k$-node tree $T$, and the goal is to determine whether $G$ has a subgraph isomorphic to $T$. We provide evidence that this problem cannot be computed significantly faster than $2^{k} \textsf{poly}(n)$, which matches the fastest algorithm known for this problem by Koutis and Williams [ICALP 2009 and TALG 2016]. Specifically, we show that if kTree can be solved in time $(2-\varepsilon)^k \textsf{poly}(n)$ for some constant $\varepsilon>0$, then Set Cover with $n'$ elements and $m'$ sets can be solved in time $(2-δ)^{n'} \textsf{poly}(m')$ for a constant $δ(\varepsilon) > 0$, which would refute the Set Cover Conjecture by Cygan et al. [CCC 2012 and TALG 2016]. Our techniques yield a new algorithm for the p-Partial Cover problem, a parameterized version of Set Cover that requires covering at least $p$ elements (rather than all elements). Its running time is $(2+\varepsilon)^p (m')^{O(1/\varepsilon)}$ for any fixed $\varepsilon>0$, which improves the previous $2.597^p \textsf{poly}(m')$-time algorithm by Zehavi [ESA 2015]. Our running time is nearly optimal, as a $(2-\varepsilon')^p \textsf{poly}(m')$-time algorithm would refute the Set Cover Conjecture.

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