Search

We characterize the extremal structure for the exact mixing time for random walks on trees $\mathcal{T}_{n,d}$ of order $n$ with diameter $d$. Given a graph $G=(V,E)$, let $H(v,\pi)$ denote the expected length of an optimal stopping rule from vertex $v$ to the stationary distributon $\pi$. We show that the quantity $\max_{G \in \mathcal{T}_{n,d} } T_{\mbox{mix}}(G) = \max_{G \in \mathcal{T}_{n,d} } \max_{v \in V} H(v,\pi)$ is achieved uniquely by the balanced double broom., Comment: 25 pages, 4 figures

We study hitting times in simple random walks on graphs, which measure the time required to reach specific target vertices. Our main result establishes a sharp lower bound for the variance of hitting times. For a simple random walk on a graph with $n$ vertices, we prove that the variance of the hitting time from a vertex $x$ to a vertex $y$, denoted $\tau_y$, is at least of the order $\mathbb{E}_x(\tau_y)^2 / \log n$. When the graph is a tree, we show that $n$ can be replaced by the graph's distance between vertices $x$ and $y$., Comment: There is an error in the Proof of Theorem 1.2