A discrete harmonic function bounded on a large portion of $\mathbb{Z}^2$ is constant
Lev Buhovsky, Alexander Logunov, Eugenia Malinnikova, Mikhail Sodin · arXiv (Cornell University) · 2017
An improvement of the Liouville theorem for discrete harmonic functions on $\mathbb{Z}^2$ is obtained. More precisely, we prove that there exists a positive constant $\varepsilon$ such that if $u$ is discrete harmonic on $\mathbb{Z}^2$ and for each sufficiently large square $Q$ centered at the origin $|u|\le 1$ on a $(1-\varepsilon)$ portion of $Q$ then $u$ is constant.