Gaps between consecutive eigenvalues for compact metric graphs

On a compact metric graph, we consider the spectrum of the Laplacian defined with a mix of standard and Dirichlet vertex conditions. A Cheeger-type lower bound on the gap lambda 2 - lambda 1 is established, with a constant that depends only on the total length of the graph and minimum edge length. We also prove some improvements of known upper bounds for eigenvalue gaps and ratios for metric trees and extensions to certain other types of graphs.(c) 2023 Elsevier Inc. All rights reserved.