Triangles without a common vertex
Year: 2026
Type: preprint
Abstract: Let $G$ be a graph with $m$ edges, $t(G)$ triangles, and adjacency spectral radius $λ(G)>\sqrt m$. Li, Feng and Peng (2025) conjectured that $t(G)\ge\sqrt m-O(1)$ if no vertex belongs to every triangle of $G$. We prove the stronger bound $t(G)\geλ(G)-C$ for an absolute constant $C$, and give an explicit choice of $C$. The proof bounds the sum of the squared adjacency eigenvalues other than the largest and smallest. Interlacing and matrix perturbation estimates then restrict the neighbourhood of a vertex corresponding to a largest entry of a Perron vector to two disjoint edges and isolated vertices. A weighted count of the triangles completes the proof.
Source: arXiv (Cornell University)
Authors: Cai, Dongxiu, Zhang, Xiao-Dong
Language: English
Subfield: Discrete Mathematics and Combinatorics
Field: Mathematics
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7225833268
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