Combinatorial Perron values of trees and bottleneck matrices

Abstract

The algebraic connectivity $a(G)$ of a graph $G$ is an important parameter, defined as the second smallest eigenvalue of the Laplacian matrix of $G$. If $T$ is a tree, $a(T)$ is closely related to the Perron values (spectral radius) of so-called bottleneck matrices of subtrees of $T$. In this setting we introduce a new parameter called the {\em combinatorial Perron value} $\rho_c$. This value is a lower bound on the Perron value of such subtrees; typically $\rho_c$ is a good approximation to $\rho$. We compute exact values of $\rho_c$ for certain special subtrees. Moreover, some results concerning $\rho_c$ when the tree is modified are established, and it is shown that, among trees with given distance vector (from the root), $\rho_c$ is maximized for caterpillars.