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Дискретная математика и математическая кибернетика
Об одном классе вершинно-транзитивных дистанционно регулярных накрытий полных графов, II
Л. Ю. Циовкина Krasovsky Institute of Mathematics and Mechanics, 16, S. Kovalevskoi str., Yekaterinburg, 620090, Russia
Аннотация:
Let
$\Gamma$ be an abelian antipodal distance-regular graph of diameter 3 with the following property:
$(*)$ $\Gamma$ has a transitive group
$\overline{G}$ of automorphisms which induces a primitive almost simple permutation group
$\overline{G}^{\Sigma}$ on the set
${\Sigma}$ of its antipodal classes. If permutation rank
${\rm rk}(\overline{G}^{\Sigma})$ of
$\overline{G}^{\Sigma}$ equals
$2$, then
$\Gamma$ is arc-transitive; moreover, all such graphs are now known. The purpose of this paper is to describe the graphs
$\Gamma$ with the property
$(*)$ in the case when
${\rm rk}(\overline{G}^{\Sigma})=3$. According to the classification of primitive almost simple permutation groups of rank
$3$ the socle of the group
$\overline{G}^{\Sigma}$ under the given condition is either a sporadic simple group, or an alternating group, or a simple group of exceptional Lie type, or a classical simple group. Earlier, we described the graphs
$\Gamma$ provided that
${\rm rk}(\overline{G}^{\Sigma})=3$ and the socle of
$\overline{G}^{\Sigma}$ is a sporadic simple group. Here we study the cases when
$(i)$ the socle of the group
$\overline{G}^{\Sigma}$ is an alternating group or
$(ii)$ $|{\Sigma}|\le 2500$ and socle of
$\overline{G}^{\Sigma}$ is a simple group of exceptional Lie type. We show that the family of non-bipartite graphs
$\Gamma$ with the property
$(*)$ and
$\mathrm{rk}(\overline{G}^{\Sigma})=3$ in the alternating case is finite and limited to a small number of potential examples with
$|\Sigma|\in\{10,28,120\}$, each of which is a covering of one of five certain distance-transitive Taylor graphs. For each given group
$\overline{G}^{\Sigma}$ of degree
$|{\Sigma}|\le 2500$ of exceptional type, we essentially restrict the set of admissible parameters of
$\Gamma$.
Ключевые слова:
distance-regular graph, antipodal cover, abelian cover, vertex-transitive graph, rank 3 group.
УДК:
512.542.7,
519.17
MSC: 05B25,
05E18 Поступила 25 марта 2022 г., опубликована
5 июля 2022 г.
DOI:
10.33048/semi.2022.19.030