Extremal spectral radius of nonregular graphs with prescribed maximum degree

Abstract

Let $G$ be a graph attaining the maximum spectral radius among all connected nonregular graphs of order $n$ with maximum degree $\Delta$. Let $\lambda_1(G)$ be the spectral radius of $G$. A nice conjecture due to Liu, Shen and Wang [On the largest eigenvalue of non-regular graphs, J. Combin. Theory Ser. B, 97 (2007) 1010--1018] asserts that \[ \lim_{n\to\infty} \frac{n^2(\Delta-\lambda_1(G))}{\Delta-1} = \pi^2 \] for each fixed $\Delta$. Concerning an important structural property of the extremal graphs $G$, Liu and Li present another conjecture which states that $G$ has degree sequence $\Delta,\ldots,\Delta,\delta$. Here, $\delta=\Delta-1$ or $\delta=\Delta-2$ depending on the parity of $n\Delta$. In this paper, we make progress on the two conjectures. To be precise, we disprove the first conjecture for all $\Delta\geq 3$ by showing that the limit superior is at most $\pi^2/2$. For small $\Delta$, we determine the precise asymptotic behavior of $\Delta-\lambda_1(G)$. In particular, we show that $\lim\limits_{n\to\infty} n^2 (\Delta - \lambda_1(G)) /(\Delta - 1) = \pi^2/4$ if $\Delta=3$; and $\lim\limits_{n\to\infty} n^2 (\Delta - \lambda_1(G)) /(\Delta - 2) = \pi^2/2$ if $\Delta = 4$. We also confirm the second conjecture for $\Delta = 3$ and $\Delta = 4$ by determining the precise structure of extremal graphs. Particularly, we show that the extremal graphs for $\Delta\in\{3,4\}$ must have a path-like structure built from specific blocks.