Finite transitive groups having many suborbits of cardinality at most two and an application to the enumeration of Cayley graphs

TL;DR

This paper classifies finite transitive groups with a high proportion of small suborbits, and applies these results to bound the number of Cayley graphs with specific automorphism groups.

Contribution

It provides a classification of transitive groups with over 5/6 of elements in small suborbits and applies this to enumerate Cayley graphs with certain automorphism properties.

Findings

If the proportion exceeds 5/6, all elements are in small suborbits and the group is classified.

The groups attaining the 5/6 bound are explicitly classified.

An upper bound on the number of Cayley graphs with a given automorphism group is established.

Abstract

Let $G$ be a finite transitive group on a set $\Omega$, let $\alpha\in \Omega$ and let $G_\alpha$ be the stabilizer of the point $\alpha$ in $G$. In this paper, we are interested in the proportion $$\frac{|\{\omega\in \Omega\mid \omega \textrm{ lies in a }G_\alpha\textrm{-orbit of cardinality at most two}\}|}{|\Omega|},$$ that is, the proportion of elements of $\Omega$ lying in a suborbit of cardinality at most two. We show that, if this proportion is greater than $5/6$, then each element of $\Omega$ lies in a suborbit of cardinality at most two and hence $G$ is classified by a result of Bergman and Lenstra. We also classify the permutation groups attaining the bound $5/6$. We use these results to answer a question concerning the enumeration of Cayley graphs. Given a transitive group $G$ containing a regular subgroup $R$, we determine an upper bound on the number of Cayley graphs on $R$ containing $G$ in their automorphism groups.