Spectrally symmetric orientations of graphs

TL;DR

This paper investigates the spectral symmetry of Hermitian adjacency matrices of oriented graphs, establishing bounds on their density, necessary conditions for symmetry, and methods for constructing such graphs.

Contribution

It introduces bounds on the density of spectrally symmetric oriented graphs and provides necessary conditions and construction methods for these graphs.

Findings

Maximum density of spectrally symmetric graphs is between 13/18 and 10/11.

Line graphs of dense graphs do not admit spectrally symmetric orientations.

Construction of infinite families of spectrally symmetric graphs using 1-sums.

Abstract

The Hermitian adjacency matrices of digraphs based on the sixth root of unity were introduced in [B. Mohar, A new kind of Hermitian matrices for digraphs, Linear Alg. Appl. (2020)]. They appear to be the most natural choice for the spectral theory of digraphs. Undirected graphs have adjacency spectrum symmetric about 0 if and only if they are bipartite. The situation is more complex for the Hermitian spectra of digraphs. In this paper we study non-bipartite oriented graphs with symmetric Hermitian spectra. Our main result concerns the extremal problem of maximizing the density of spectrally symmetric oriented graphs. The maximum possible density is shown to be between 13/18} and 10/11. Furthermore, we give a necessary condition for an oriented graph to be spectrally symmetric based on the adjacency spectrum of the underlying graph. This allows us to show that line graphs of sufficiently dense graphs do not admit spectrally symmetric orientations. We also show how to construct infinite families of spectrally symmetric graphs using 1-sums.