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## Acta Arithmetica

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## Structural properties and formulae of the spectra of integral circulant graphs

### Volume 184 / 2018

Acta Arithmetica 184 (2018), 297-315 MSC: Primary 05C50; Secondary 11L03. DOI: 10.4064/aa171020-30-6 Published online: 3 August 2018

#### Abstract

Every integral circulant graph on $n$ vertices is isomorphic to some graph $\newcommand{\Icg}[2]{\mathrm{ICG}({#2},{#1})}\Icg{\mathcal D}{n}$ having vertex set $\mathbb{Z}/n\mathbb{Z}$ and edge set $\{(a,b):\, a,b\in\mathbb{Z}/n\mathbb{Z} ,\, \gcd(a-b,n)\in {\cal D}\}$ for a uniquely determined set $\mathcal D$ of positive divisors of $n$. According to a conjecture of So, two integral circulant graphs are isomorphic if and only if their spectra, i.e. the eigenvalues of their adjacency matrices, coincide. In order to facilitate a deeper understanding of the interrelation between integral circulant graphs and their spectra, we deduce several structural spectral properties of $\newcommand{\Icg}[2]{\mathrm{ICG}({#2},{#1})}\Icg{\mathcal D}{p^k}$ with prime power order $p^k$ and establish an explicit parameterisation of the spectrum of $\newcommand{\Icg}[2]{\mathrm{ICG}({#2},{#1})}\Icg{\mathcal D}{n}$ for multiplicative divisor sets $\mathcal D$. Our crucial tool will be the new concept of the leaping set of $\mathcal D$.

#### Authors

• J. W. SanderInstitut für Mathematik und Angewandte Informatik
Universität Hildesheim
D-31141 Hildesheim, Germany
e-mail

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