On alpha-adic expansions in Pisot bases

  • Authors:
  • Petr Ambro;Christiane Frougny

  • Affiliations:
  • Doppler Institute and Department of Mathematics, FNSPE, Czech Technical University, Czech Republic and LIAFA, UMR 7089 CNRS & Universitéé Paris 7, France;LIAFA, UMR 7089 CNRS & Universitéé Paris 7, France and Université Paris 8, France

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2007

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Abstract

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