L'accès aux articles de la revue « Journal de Théorie des Nombres de Bordeaux » (http://jtnb.cedram.org/), implique l'accord avec les conditions générales d'utilisation (http://jtnb.cedram. org/legal/). Toute reproduction en tout ou partie cet article sous quelque forme que ce soit pour tout usage autre que l'utilisation à fin strictement personnelle du copiste est constitutive d'une infraction pénale. Toute copie ou impression de ce fichier doit contenir la présente mention de copyright. Abstract. For abstract numeration systems built on exponential regular languages (including those coming from substitutions), we show that the set of real numbers having an ultimately periodic representation is Q(β) if the dominating eigenvalue β > 1 of the automaton accepting the language is a Pisot number. Moreover, if β is neither a Pisot nor a Salem number, then there exist points in Q(β) which do not have any ultimately periodic representation.
IntroductionIn [7], abstract numeration systems on regular languages have been introduced. They generalize in a natural way a large variety of classical positional systems like the q-ary system or the Fibonacci system: each nonnegative integer n is represented by the nth word of an ordered infinite regular language L. For instance, considering the natural ordering of the digits, the genealogical enumeration of the words belonging to the language L = {0}∪{1, . . . , q−1}{0, . . . , q−1} * (resp. L = {0, 1}∪{10}{10, 0} * {λ, 1}) leads back to the q-ary (resp. the Fibonacci) system. Later on, this setting has been extended to allow the representation of real numbers as well as of integers.Various notions appearing in number theory, in formal languages theory or in the analysis of algorithms depend on how numbers are represented.The second author was supported by the Austrian Science Foundation FWF, grant S8302-MAT.
284Michel Rigo, Wolfgang Steiner So these abstract systems have led to new nontrivial applications. To cite just a few: the characterization of the so-called recognizable sets of integers, the investigation of the dynamical and topological properties of the "odometers" or the study of the asymptotic behavior of the corresponding "sum-of-digits" function.As we will see, it turns out that this way of representing real numbers is a quite natural generalization of Rényi's β-expansions [11]. More precisely, the primitive automata lead to the representations of real numbers based on substitutions introduced by Dumont and Thomas [3]. The nonprimitive automata provide new numeration systems.Real numbers having an ultimately periodic representation deserve a special attention. Indeed, for the q-ary system, these numbers are exactly the rational numbers. More generally, the set of ultimately periodic representations is dense in the set of all the admissible representations and therefore this rises number-theoretic questions like the approximation of real numbers by numbers having ultimately periodic expansions.On the one hand, for Rényi's classical β-expansions it is well-known that t...