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arXiv 1806.03462math.CO
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参数为$(n,k,k-1,a)$和$β=1$的Deza图

Deza graphs with parameters $(n,k,k-1,a)$ and $β=1$

Sergey Goryainov, Willem H. Haemers, Vladislav V. Kabanov, Leonid Shalaginov

摘要:

一个带参数$(n,k,b,a)$的Deza图是一个具有$n$顶点的$k$ -正则图,其中任意两个顶点有$a$或$b$($a\leq b$)公共邻居.一个Deza图是严格Deza图,如果它有直径$2$,并且不是强正则的.在早期的论文中,最后两位作者等人。刻画了具有$b=k-1$和$β> 1$的严格Deza图,其中$β$是与给定顶点具有$b$公共邻居的顶点数.本文讨论了$β=1$的情形,从而完成了严格Deza图的$b=k-1$刻画.因此,所有具有$b=k-1$和$β=1$的Deza图都可以由特殊的强正则图构成,并且我们给出了几个这样的强正则图的例子.可分设计图是一种特殊的Deza图,具有$β=1$的Deza图是可分设计图.目前的表征揭示了一个错误的可分设计图的第二作者等人的文件中,我们讨论了这个错误的原因和后果,并给出所需的勘误表。

英文摘要:

A Deza graph with parameters $(n,k,b,a)$ is a $k$-regular graph with $n$ vertices in which any two vertices have $a$ or $b$ ($a\leq b$) common neighbours. A Deza graph is strictly Deza if it has diameter $2$, and is not strongly regular. In an earlier paper, the two last authors et el. characterized the strictly Deza graphs with $b=k-1$ and $β> 1$, where $β$ is the number of vertices with $b$ common neighbours with a given vertex. Here we deal with the case $β=1$, thus we complete the characterization of strictly Deza graphs with $b=k-1$. It follows that all Deza graphs with $b=k-1$ and $β=1$ can be made from special strongly regular graphs, and we present several examples of such strongly regular graphs. A divisible design graph is a special Deza graph, and a Deza graph with $β=1$ is a divisible design graph. The present characterization reveals an error in a paper on divisible design graphs by the second author et al. We discuss the cause and the consequences of this mistake and give the required errata.

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