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arXiv · 2607.02951

Signed graphs with fixed smallest eigenvalue at least $-3$ and their lattices

Meng-Yue Cao·Jack H. Koolen·Jing-Yuan Liu·Qianqian Yang
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Abstract

In this paper, we consider connected signed graphs with smallest eigenvalue at least $-3-\varepsilon$ for a small positive constant $\varepsilon$. We prove that if such a signed graph has sufficiently large minimum valency, then its smallest eigenvalue is at least $-3$, and the lattice associated with it, which is generated by squared norm $3$ vectors, is a sublattice of a direct sum of the standard lattice $\mathbb{Z}^n$ and copies of the root lattice $E_8$. Moreover, there exist infinitely many connected signed graphs with smallest eigenvalue at least $-3$ containing it as a proper induced subgraph. Furthermore, we discuss signed graphs with smallest eigenvalue $-3$ arising from rootless irreducible unimodular lattices.

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Meng-Yue Cao, Jack H. Koolen, Jing-Yuan Liu, Qianqian Yang. 2026-07-10. Signed graphs with fixed smallest eigenvalue at least $-3$ and their lattices. https://arxiv.org/abs/2607.02951

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