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arXiv 1208.3067math.CO
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强行走-正则图

Strongly walk-regular graphs

Edwin R. van Dam, Gholamreza Omidi

摘要:

研究了强正则图的一个推广。我们称一个图为强行走正则的,如果存在一个$\ell >1$使得从一个顶点到另一个顶点的长度为$\ell$的行走的数目仅取决于这两个顶点是相同的、相邻的还是不相邻的。我们将证明一个强行走正则图必须是空图、完全图、强正则图、相同大小的孤立点的完全二部图的不交并,或具有四个特征值的正则图。这个列表中的前三个族的图确实是强$\ell$ -行走正则的,对于所有$\ell$,而第四个族的图是$\ell$ -行走正则的,对于每个奇数$\ell$。具有四个特征值的正则图的情况是最有趣(也是最复杂)的。这样的图即使对于$\ell$也不能是强$\ell$ -行走正则的。我们用特征值刻画了正则四特征值图对每一个奇$\ell$都是强$\ell$ -行走正则的情形。有几个这样的图的无限族的例子。我们将证明每隔一个正则四特征值图都可以是强$\ell$ -行走正则的,至多有一个$\ell$。有几个这样的图的无限族是强3-行走正则的例子。然而,是否有任何图对于仅一个不同于3的特定$\ell$是强$\ell$ -行走-正则的仍然是开放的。

英文摘要:

We study a generalization of strongly regular graphs. We call a graph strongly walk-regular if there is an $\ell >1$ such that the number of walks of length $\ell$ from a vertex to another vertex depends only on whether the two vertices are the same, adjacent, or not adjacent. We will show that a strongly walk-regular graph must be an empty graph, a complete graph, a strongly regular graph, a disjoint union of complete bipartite graphs of the same size and isolated vertices, or a regular graph with four eigenvalues. Graphs from the first three families in this list are indeed strongly $\ell$-walk-regular for all $\ell$, whereas the graphs from the fourth family are $\ell$-walk-regular for every odd $\ell$. The case of regular graphs with four eigenvalues is the most interesting (and complicated) one. Such graphs cannot be strongly $\ell$-walk-regular for even $\ell$. We will characterize the case that regular four-eigenvalue graphs are strongly $\ell$-walk-regular for every odd $\ell$, in terms of the eigenvalues. There are several examples of infinite families of such graphs. We will show that every other regular four-eigenvalue graph can be strongly $\ell$-walk-regular for at most one $\ell$. There are several examples of infinite families of such graphs that are strongly 3-walk-regular. It however remains open whether there are any graphs that are strongly $\ell$-walk-regular for only one particular $\ell$ different from 3.

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