# Existence of non-Cayley Haar graphs

## TL;DR

This paper proves that most finite non-abelian groups have Haar graphs that are not Cayley graphs, except for a few specific groups, resolving an open problem from 2016.

## Contribution

It demonstrates the existence of non-Cayley Haar graphs for all finite non-abelian groups except a small set of exceptions, answering a previously open question.

## Key findings

- Most finite non-abelian groups admit non-Cayley Haar graphs.
- The exceptions are the dihedral groups D6, D8, D10, the quaternion group Q8, and Q8×Z2.
- This resolves an open problem in the theory of Haar graphs.

## Abstract

A Cayley graph of a group $H$ is a finite simple graph $\Gamma$ such that its automorphism group ${\rm Aut}(\Gamma)$ contains a subgroup isomorphic to $H$ acting regularly on $V(\Gamma)$, while a Haar graph of $H$ is a finite simple bipartite graph $\Sigma$ such that ${\rm Aut}(\Sigma)$ contains a subgroup isomorphic to $H$ acting semiregularly on $V(\Sigma)$ and the $H$-orbits are equal to the partite sets of $\Sigma$. It is well-known that every Haar graph of finite abelian groups is a Cayley graph. In this paper, we prove that every finite non-abelian group admits a non-Cayley Haar graph except the dihedral groups $D_6$, $D_8$, $D_{10}$, the quaternion group $Q_8$ and the group $Q_8\times\mathbb{Z}_2$. This answers an open problem proposed by Est\'elyi and Pisanski in 2016.