Characterization of rational matrices that admit finite digit representations

Abstract

Let $A$ be an $n \times n$ matrix with rational entries and let \[ \mathbb{Z}^n[A] := \bigcup_{k=1}^{\infty} \left( \mathbb{Z}^n + A\mathbb{Z}^n + \dots + A^{k-1}\mathbb{Z}^n\right) \] be the minimal $A$-invariant $\mathbb{Z}$-module containing the lattice $\mathbb{Z}^n$. If $\mathcal{D}\subset\mathbb{Z}^n[A]$ is a finite set we call the pair $(A,\mathcal{D})$ a digit system. We say that $(A,\mathcal{D})$ has the finiteness property if each $\mathbf{z} \in \mathbb{Z}^n[A]$ can be written in the form \[ \mathbf{z} = \mathbf{d}_0 + A\mathbf{d}_1 + \dots + A^k\mathbf{d}_k, \] with $k\in\mathbb{N}$ and digits $\mathbf{d}_j \in \mathcal{D}$ for $0\le j\le k$. We prove that for a given matrix $A \in M_n(\mathbb{Q})$ there is a finite set $\mathcal{D}\subset\mathbb{Z}^n[A]$ such that $(A, \mathcal{D})$ has the finiteness property if and only if $A$ has no eigenvalue of absolute value $< 1$. This result is the matrix analogue of the height reducing property of algebraic numbers. In proving this result we also characterize integer polynomials $P \in \mathbb{Z}[x]$ that admit digit systems having the finiteness property in the quotient ring $\mathbb{Z}[x]/(P)$.