The spectral radius of subgraphs of regular graphs

We give a bound on the spectral radius of subgraphs of regular graphs with given order and diameter. We give a lower bound on the smallest eigenvalue of a nonbipartite regular graph of given order and diameter

On the spectra and spectral radii of token graphs

Let $G$ be a graph on $n$ vertices. The $k$-token graph (or symmetric $k$-th power) of $G$, denoted by $F_k(G)$ has as vertices the ${n\choose k}$ $k$-subsets of vertices from $G$, and two vertices are adjacent when their symmetric difference is a pair of adjacent vertices in $G$. In particular, $F_k(K_n)$ is the Johnson graph $J(n,k)$, which is a distance-regular graph used in coding theory. In this paper, we present some results concerning the (adjacency and Laplacian) spectrum of $F_k(G)$ in terms of the spectrum of $G$. For instance, when $G$ is walk-regular, an exact value for the spectral radius $\rho$ (or maximum eigenvalue) of $F_k(G)$ is obtained. When $G$ is distance-regular, other eigenvalues of its $2$-token graph are derived using the theory of equitable partitions. A generalization of Aldous' spectral gap conjecture (which is now a theorem) is proposed

The spectral radius of subgraphs of regular graphs

Let μ (G) and μmin (G) be the largest and smallest eigenvalues of the adjacency-matrix of a graph G. Our main results are: (i) Let G be a regular graph of order n and finite diameter D. If H is a proper subgraph of G, then μ(G)-μ(H) \u3e 1/nD. (ii) If G is a regular nonbipartite graph of order n and finite diameter D, then μ (G) +μmin (G) 1/nD