Laplacian spread of graphs: Lower bounds and relations with invariant parameters

The spread of an n×nn×n complex matrix B   with eigenvalues β1,β2,…,βnβ1,β2,…,βn is defined bys(B)=maxi,j⁡|βi−βj|, where the maximum is taken over all pairs of eigenvalues of B. Let G be a graph on n vertices. The concept of Laplacian spread of G is defined by the difference between the largest and the second smallest Laplacian eigenvalue of G. In this work, by combining old techniques of interlacing eigenvalues and rank 1 perturbation matrices new lower bounds on the Laplacian spread of graphs are deduced, some of them involving invariant parameters of graphs, as it is the case of the bandwidth, independence number and vertex connectivity.