Anjelo Gabriel R. Cruz

arXiv2026

A Matrix Analogue of Rational Number Systems

Let $P,Q \in \mathbb{Z}^{d\times d}$ be invertible coprime matrices such that all the eigenvalues of $Q^{-1}P$ have modulus greater than 1, and $\mathbb{Z}^d[Q^{-1}P]$ be the smallest non-trivial $Q^{-1}P$-invariant $\mathbb{Z}$-module containing $\mathbb{Z}^d$. Suppose there is a finite digit set $\mathcal{D}\subseteq \mathbb{Z}^d + P\mathbb{Z}^d[Q^{-1}P]$ for which every vector $x \in \mathbb{Z}^d + P\mathbb{Z}^d[Q^{-1}P]$ can be represented in the form \[ x = \sum_{i=0}^{\ell-1} (Q^{-1}P)^i Q^{-1}d_i, \] where the digits $d_i \in \mathcal{D}$ for all $i \in \{0,1,\ldots,\ell-1\}$. We call such a representation a $P/Q$-expansion of $x$, and we say that the digit system $(P,Q,\mathcal{D})$ has the finiteness property. If, in addition, $\mathcal{D}$ is a complete set of residues of the quotient group $(\mathbb{Z}^d + P\mathbb{Z}^d[Q^{-1}P])/P\mathbb{Z}^d[Q^{-1}P]$, then the digits $d_0, d_1, \dots, d_{\ell-1}$ in the $P/Q$-expansion of $x$ are unique whenever $\ell \in \mathbb{Z}^+$ is minimal, and the resulting digit system is said to have the uniqueness property. We present sufficient conditions for the existence of a digit set $\mathcal{D}$ in which $(P,Q,\mathcal{D})$ has the finiteness property. For $d=2$, we make use of finite automata to construct digit systems $(P,Q,\mathcal{D})$ having both the finiteness and uniqueness properties. We also obtain the $P/Q$-expansion of a vector $x$ in $\mathbb{R}^d$ by means of the so-called expansion tree of the digit system $(P,Q,\mathcal{D})$.

math.NT↗

arXiv2025

Addition Automata and Attractors of Digit Systems Corresponding to Expanding Rational Matrices

Let $A$ be an expanding $2 \times 2$ matrix with rational entries and $\mathbb{Z}^2[A]$ be the smallest $A$-invariant $\mathbb{Z}$-module containing $\mathbb{Z}^2$. Let $\mathcal{D}$ be a finite subset of $\mathbb{Z}^2[A]$ which is a complete residue system of $\mathbb{Z}^2[A]/A\mathbb{Z}^2[A]$. The pair $(A,\mathcal{D})$ is called a {\em digit system} with {\em base} $A$ and {\em digit set} $\mathcal{D}$. It is well known that every vector $x \in \mathbb{Z}^2[A]$ can be written uniquely in the form \[ x = d_0 + Ad_1 + \cdots + A^kd_k + A^{k+1}p, \] with $k\in \mathbb{N}$ minimal, $d_0,\dots,d_k \in \mathcal{D}$, and $p$ taken from a finite set of {\em periodic elements}, the so-called {\em attractor} of $(A,\mathcal{D})$. If $p$ can always be chosen to be $0$ we say that $(A,\mathcal{D})$ has the {\em finiteness property}. In the present paper we introduce finite-state transducer automata which realize the addition of the vectors $\pm(1,0)^\top$ and $\pm(0,1)^\top$ to a given vector $x\in \mathbb{Z}^2[A]$ in a number system $(A,\mathcal{D})$ with collinear digit set. These automata are applied to characterize all pairs $(A,\mathcal{D})$ that have the finiteness property and, more generally, to characterize the attractors of these digit systems.

math.NT↗