AI中文摘要
在本文中,我们展示了边添加或删除对连通图$G$的第二大特征值$\lambda_2(G)$的影响。1989年,Chung、Graham和Wilson证明了对于阶数为$n$的稠密无$K_{r+1}$图,有$\max\{|\lambda_2|,|\lambda_n|\}>\Omega(n)$,这从谱角度理解了大团或大独立集的存在性,与Ramsey理论相关。应用边操作对$\lambda_2$影响的结果,我们确定了所有给定阶数的无$K_{r+1}$连通图中$\lambda_2$的最大值,并完全刻画了极图。此外,对于任意给定的图$F$,我们研究了阶数为$n$的无$F$连通图中第二大特征值$\lambda_2(G)$的最大值。设$\rho^*(n,F)$为$n\ge n_F$个顶点的无$F$图的最大谱半径,$G^*(n,F)$为满足谱半径$\rho\big(G^*(n,F)\big)=\rho^*(n,F)$的图。我们证明,对于阶数为$n\ge f(n_F)$的无$F$连通图$G$,(1) 若$n$为奇数,则$$\lambda_2(G)\le\rho^*\left(\frac{n-1}{2},F\right)$$等号成立当且仅当$G\in \mathcal{I}\big(G^*(\frac{n-1}{2},F),G^*(\frac{n-1}{2},F)\big)$;(2) 若$n$为偶数,且$F$不含割边,则具有最大第二大特征值的图$G^†$满足$$\lambda_2(G^†)=\rho^*\left(\frac{n}{2},F\right)-o(1)$$且$G^†\in \mathcal{E}\big(H_1,H_2\big)$,其中$H_1$和$H_2$是$\frac{n}{2}$个顶点上的$F$-饱和图。特别地,除了完全图$K_{r+1}$外,当$F$为书图$B_{k+1}$或奇圈$C_{2k+1}$时,我们能够确定给定阶数的无$F$连通图中第二大特征值的最大值,并完全刻画极图。
英文摘要
In this paper, we demonstrate the effects on the second largest eigenvalue $λ_2(G)$ of a connected graph $G$ after edge addition or deletion.
In 1989, Chung, Graham and Wilson showed $\max\{|λ_2|,|λ_n|\}>Ω(n)$ for dense $K_{r+1}$-free graphs of order $n$, giving spectral comprehension of existence of large clique or independent set, respect to Ramsey theory. Applying the results of effects on $λ_2$ after edge operations, we determine the maximum value of $λ_2$ among all $K_{r+1}$-free connected graphs with given order, and completely characterize the extremal graphs.
Moreover, for arbitrary given graph $F$, we investigates the maximum second largest $λ_2(G)$ among $F$-free connected graphs of order $n$. Let $ρ^*(n,F)$ be the maximum spectral radius of $F$-free graphs on $n\ge n_F$ vertices, and $G^*(n,F)$ be a graph with its spectral radius $ρ\big(G^*(n,F)\big)=ρ^*(n,F)$. We prove that, for an $F$-free connected graph $G$ of order $n\ge f(n_F)$, \\(1) if $n$ is odd, then $$λ_2(G)\leρ^*\left(\frac{n-1}{2},F\right)$$ with equality if and only if $G\in \mathcal{I}\big(G^*(\frac{n-1}{2},F),G^*(\frac{n-1}{2},F)\big)$; and\\ (2) if $n$ is even, and $F$ does not contain cut edges, then the graph $G^†$ with the maximum second largest eigenvalue satisfies $$λ_2(G^†)=ρ^*\left(\frac{n}{2},F\right)-o(1)$$ and $G^†\in \mathcal{E}\big(H_1,H_2\big)$, where $H_1$ and $H_2$ are $F$-saturated graphs on $\frac{n}{2}$ vertices.
In particular, other than a complete graph $K_{r+1}$, when $F$ is a book graph $B_{k+1}$ or an odd cycle $C_{2k+1}$, we are able to determine the maximum second largest eigenvalue for $F$-free connected graphs of given order, and completely characterize the extremal graphs.