The spectral Tur\'{a}n problem: Characterizing spectral-consistent graphs

TL;DR

This paper characterizes when graphs are spectral-consistent, showing that certain structural conditions on the forbidden graph ensure spectral extremal graphs are also size extremal, thus broadening understanding in spectral extremal graph theory.

Contribution

The paper introduces a weaker structural condition, matching-goodness of the decomposition family, that guarantees spectral-consistency for a wide class of graphs, expanding previous results.

Findings

Established that matching-good decomposition families imply spectral-consistency.

Fully characterized spectral-consistency for several important graph families.

Provided a simplified proof of an existing spectral-consistency result.

Abstract

Let ${\rm EX}(n,H)$ and ${\rm SPEX}(n,H)$ denote the families of $n$-vertex $H$-free graphs with the maximum size and the maximum spectral radius, respectively. A graph $H$ is said to be spectral-consistent if ${\rm SPEX}(n,H)\subseteq {\rm EX}(n,H)$ for sufficiently large $n$. A fundamental problem in spectral extremal graph theory is to determine which graphs are spectral-consistent. Cioab\u{a}, Desai and Tait [European J. Combin. 99 (2022) 103420] proposed the following conjecture: Let $H$ be any graph such that the graphs in ${\rm EX}(n,H)$ are Tur\'{a}n graph plus $O(1)$ edges. Then $H$ is spectral-consistent. Wang, Kang and Xue [J. Combin. Theory Ser. B 159 (2023) 20--41] confirmed this conjecture, along with a stronger result. Recently, Liu and Ning raised a general problem in spectral extremal graph theory: Characterize all graphs that are spectral-consistent. In this paper, we establish that for any finite graph \(H\), if its decomposition family is matching-good, then \(H\) is necessarily spectral-consistent. Notably, this structural condition is strictly weaker than the condition for spectral-consistency established by Wang, Kang, and Xue in their earlier work, thereby broadening the class of graphs known to satisfy the spectral-consistency property. Our main result enables us to fully characterize the spectral-consistency for several important families of forbidden graphs \(H\), including generalized color-critical graphs, odd-ballooning of trees and complete bipartite graphs, as well as edge blow-up of non-bipartite graphs and certain special bipartite graphs. Furthermore, we present a streamlined proof for an existing spectral-consistency result due to Chen, Lei, and Li, simplifying their original argument. Finally, we propose several open problems to motivate future research in this area.