Minimal harmonic measure on 2D lattices

TL;DR

This paper investigates the asymptotic behavior of the minimal harmonic measure on three 2D lattices, confirming conjectures and extending previous exponential decay results for various graphs.

Contribution

It provides precise bounds for the minimal harmonic measure on square, triangular, and hexagonal lattices, confirming a stronger version of a recent conjecture and extending prior exponential decay results.

Findings

Bounds for harmonic measure decay rates on lattices

Confirmation of a conjecture on harmonic measure asymptotics

Extension of exponential decay results to multiple lattice types

Abstract

We study the harmonic measure (i.e. the limit of the hitting distribution of a simple random walk starting from a distant point) on three canonical two-dimensional lattices: the square lattice $\mathbb{Z}^2$, the triangular lattice $\mathscr{T}$ and the hexagonal lattice $\mathscr{H}$. In particular, for the least positive value of the harmonic measure of any $n$-point set, denoted by $\mathscr{M}_n(\mathscr{G})$, we prove in this paper that $$[\lambda(\mathscr{G})]^{-n+c\sqrt{n}} \le \mathscr{M}_n(\mathscr{G})\le [\lambda(\mathscr{G})]^{-n+C\sqrt{n}},$$ where $\lambda(\mathbb{Z}^2)=(2+\sqrt{3})^2$, $\lambda(\mathscr{T})=3+2\sqrt{2}$ and $\lambda(\mathscr{H})=(\tfrac{3+\sqrt{5}}{2})^3$. Our results confirm a stronger version of the conjecture proposed by Calvert, Ganguly and Hammond (2023) which predicts the asymptotic of the exponent of $\mathscr{M}_n(\mathbb{Z}^2)$. Moreover, these estimates also significantly extend the findings in our previous paper with Kozma (2023) that $\mathscr{M}_n(\mathscr{G})$ decays exponentially for a large family of graphs $\mathscr{G}$ including $\mathscr{T}$, $\mathscr{H}$ and $\mathbb{Z}^d$ for all $d\ge 2$.