A conjecture about spectral distances between cycles, paths and certain trees

Abstract

We confirm the following conjecture which has been proposed in [{\em Linear Algebra and its Applications}, {\bf 436} (2012), No. 5, 1425-1435.]: $$ 0.945\approx\displaystyle\lim_{n\longrightarrow \infty}\sigma(P_n,Z_n)=\displaystyle\lim_{n\longrightarrow \infty}\sigma(W_n,Z_n)=\frac{1}{2}\displaystyle\lim_{n\longrightarrow \infty}\sigma(P_n,W_n);\ \displaystyle\lim_{n\longrightarrow \infty}\sigma(C_{2n},Z_{2n})=2,$$ where $\sigma(G_1,G_2)=\sum_{i=1}^n |\lambda_i(G_1)-\lambda_i(G_2)|$ is the spectral distance between $n$ vertex non-isomorphic graphs $G_1$ and $G_2$ with adjacency spectra $\lambda_1(G_i) \geq \lambda_2(G_i) \geq \cdots \geq \lambda_n(G_i)$ for $i=1,2$, and $P_n$ and $C_n$ denote the path and cycle on $n$ vertices, respectively; $Z_n$ denotes the coalescence of $P_{n-2}$ and $P_3$ on one of the vertices of degree 1 of $P_{n-2}$ and the vertex of degree $2$ of $P_3$; and $W_n$ denotes the coalescence of $Z_{n-2}$ and $P_3$ on the vertex of degree 1 of $Z_{n-2}$ which is adjacent to a vertex of degree $2$ and the vertex of degree $2$ of $P_3$.