Rational numbers with purely periodic $\beta$-expansion

Boris Adamczewski (1) , Christiane Frougny (2) , Anne Siegel (3) , Wolfgang Steiner (2)

1 ICJ - Institut Camille Jordan
2 LIAFA - Laboratoire d'informatique Algorithmique : Fondements et Applications
3 SYMBIOSE - Biological systems and models, bioinformatics and sequences

We study real numbers $\beta$ with the curious property that the $\beta$-expansion of all sufficiently small positive rational numbers is purely periodic. It is known that such real numbers have to be Pisot numbers which are units of the number field they generate. We complete known results due to Akiyama to characterize algebraic numbers of degree $3$ that enjoy this property. This extends results previously obtained in the case of degree $2$ by Schmidt, Hama and Imahashi. Let $\gamma(\beta)$ denote the supremum of the real numbers $c$ in $(0,1)$ such that all positive rational numbers less than $c$ have a purely periodic $\beta$-expansion. We prove that $\gamma(\beta)$ is irrational for a class of cubic Pisot units that contains the smallest Pisot number $\eta$. This result is motivated by the observation of Akiyama and Scheicher that $\gamma(\eta)=0.666 666 666 086 \cdots$ is surprisingly close to $2/3$.

Domaines: Théorie des nombres [math.NT]; Systèmes dynamiques [math.DS]

Boris Adamczewski, Christiane Frougny, Anne Siegel, Wolfgang Steiner. Rational numbers with purely periodic $\beta$-expansion. Bulletin of the London Mathematical Society, 2010, 42 (3), pp.538-552. ⟨10.1112/blms/bdq019⟩. ⟨hal-00400799v2⟩