On the Dimension of the Space of Harmonic Functions on a Discrete Torus

Abstract

Let $d(n)$ denote the corank of $I + A$ over the field with two elements, where $A$ is the adjacency matrix of the discrete torus $C_n × C_n$, and $I$ is the identity matrix. We shall prove that $d(2n) = 2d(n)$ and $d(2^r + 1) = d(2^r − 1) + 4$. For the proof of the latter result, we use an elliptic curve. Our motivation for this study is the “lights out” puzzle.