arXiv 1504.028762015-04-11cs.DS

子图同构的困难性

The Hardness of Subgraph Isomorphism

Marek Cygan, Jakub Pachocki, Arkadiusz Socała

摘要：

子图同构是一个非常基本的图问题，其中给定两个图$G$和$H$，一个是检验$G$是否是$H$的子图。尽管它的定义很简单，但子图同构问题是非常广泛的，因为它推广了诸如团，$r$ -着色，Hamilton性，集合包装和带宽等问题。然而，对于所有提到的问题都存在$2^{\mathcal{O}(n)}$时间算法，因此过去一个自然且经常被问到的问题是是否存在子图同构的$2^{\mathcal{O}(n)}$时间算法。在Fomin和Kratsch [Springer'10]的专着中，这个问题被强调为一个开放的问题。我们的主要结果是减少从3-SAT，产生一个次指数数量的次线性实例的子图同构问题。特别地，我们的约简隐含了指数时间假设下子图同构的$2^{Ω(n \sqrt{\log n})}$下界。这表明，存在一类图，严格难以嵌入比团或哈密顿圈。我们减少的核心包括两个步骤。首先，我们预处理和包装变量和条款的3-SAT公式成对数大小的组。然而，分组不是任意的，因为作为结果，我们只获得组之间的有限的相互作用。在第二步中，我们克服了技术上的困难，编码评价的排列由一个简单的，但富有成效的方案猜测的大小任意映射的原像，减少的情况下，任意映射双射。事实上，当应用这一步，最近的独立结果Fomin等人。[arXiv：1502.05447（2015）]，他们证明了图同态的硬度，我们可以将他们的硬度结果转移到子图同构，这意味着$2^{Ω(n \log n / \log \log n)}$的一个几乎紧的下界。

英文摘要：

Subgraph Isomorphism is a very basic graph problem, where given two graphs $G$ and $H$ one is to check whether $G$ is a subgraph of $H$. Despite its simple definition, the Subgraph Isomorphism problem turns out to be very broad, as it generalizes problems such as Clique, $r$-Coloring, Hamiltonicity, Set Packing and Bandwidth. However, for all of the mentioned problems $2^{\mathcal{O}(n)}$ time algorithms exist, so a natural and frequently asked question in the past was whether there exists a $2^{\mathcal{O}(n)}$ time algorithm for Subgraph Isomorphism. In the monograph of Fomin and Kratsch [Springer'10] this question is highlighted as an open problem, among few others. Our main result is a reduction from 3-SAT, producing a subexponential number of sublinear instances of the Subgraph Isomorphism problem. In particular, our reduction implies a $2^{Ω(n \sqrt{\log n})}$ lower bound for Subgraph Isomorphism under the Exponential Time Hypothesis. This shows that there exist classes of graphs that are strictly harder to embed than cliques or Hamiltonian cycles. The core of our reduction consists of two steps. First, we preprocess and pack variables and clauses of a 3-SAT formula into groups of logarithmic size. However, the grouping is not arbitrary, since as a result we obtain only a limited interaction between the groups. In the second step, we overcome the technical hardness of encoding evaluations as permutations by a simple, yet fruitful scheme of guessing the sizes of preimages of an arbitrary mapping, reducing the case of arbitrary mapping to bijections. In fact, when applying this step to a recent independent result of Fomin et al.[arXiv:1502.05447 (2015)], who showed hardness of Graph Homomorphism, we can transfer their hardness result to Subgraph Isomorphism, implying a nearly tight lower bound of $2^{Ω(n \log n / \log \log n)}$.

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