On alpha-adic expansions in Pisot bases

Abstract

We study $\alpha$-adic expansions of numbers in an extension field, that is to say, left infinite representations of numbers in the positional numeration system with the base $\alpha$, where $\alpha$ is an algebraic conjugate of a Pisot number $\beta$. Based on a result of Bertrand and Schmidt, we prove that a number belongs to $\mathbb{Q}(\alpha)$ if and only if it has an eventually periodic $\alpha$-expansion. Then we consider $\alpha$-adic expansions of elements of the extension ring $\mathbb{Z}[\alpha^{-1}]$ when $\beta$ satisfies the so-called Finiteness property (F). In the particular case that $\beta$ is a quadratic Pisot unit, we inspect the unicity and/or multiplicity of $\alpha$-adic expansions of elements of $\mathbb{Z}[\alpha^{-1}]$. We also provide algorithms to generate $\alpha$-adic expansions of rational numbers.