A finite threshold for the double-comet conjecture

Abstract

For a tree $T$, let $g(T)=\lambda_1(T)-\lambda_2(T)$ be the difference between its two largest adjacency eigenvalues. A balanced double comet is obtained by attaching equally many leaves to the two endpoints of a path. Jovovi\'c, Koledin and Stani\'c conjectured that such a tree attains the minimum adjacency spectral gap among trees of any fixed order. We prove that every minimizing tree of order $n\ge200$ is a balanced double comet. We also show that, for any finite irreducible reversible continuous-time Markov chain, the inverse spectral gap differs from the effective resistance between two states times the stationary variance of their hitting probability by at most the inverse Dirichlet gap for killing at those states. For Perron chains, we give an exact two-vertex Schur-complement formula for this resistance--variance quantity.