Two applications of the spectrum of numbers

Let the base $\beta$ be a complex number, $\mathrm{\mid }\beta \mathrm{\mid }>1$, and let $A\subset \text{C}$ be a finite alphabet of digits. The \emph{$A$-spectrum} of $\beta$ is the set $S_{A}(\beta) = {\sum_{k=0}^n a_k\beta^k \mid n \in $\mathbb{N}$, \ a_k \in {A}}$\mathrm{.}Weshowthatthespectrum$S_{{A}}(\beta)$ has an accumulation point if and only if $0$ has a particular $(\beta ,A)$-representation, said to be \emph{rigid}. The first application is restricted to the case that $\beta >1$ and the alphabet is $A=\{-M,\dots ,M\}$, $M\ge 1$ integer. We show that the set $Z_{\beta ,M}$ of infinite $(\beta ,A)$-representations of $0$ is recognizable by a finite B"uchi automaton if and only if the spectrum $S_A(\beta )$ has no accumulation point. Using a result of Akiyama-Komornik and Feng, this implies that $Z_{\beta ,M}$ is recognizable by a finite B"uchi automaton for any positive integer $M\ge \lceil \beta \rceil -1$ if and only if $\beta$ is a Pisot number. This improves the previous bound $M\ge \lceil \beta \rceil$. For the second application the base and the digits are complex. We consider the on-line algorithm for division of Trivedi and Ercegovac generalized to a complex numeration system. In on-line arithmetic the operands and results are processed in a digit serial manner, starting with the most significant digit. The divisor must be far from $0$, which means that no prefix of the $(\beta ,A)$-representation of the divisor can be small. The numeration system $(\beta ,A)$ is said to \emph{allow preprocessing} if there exists a finite list of transformations on the divisor which achieve this task. We show that $(\beta ,A)$ allows preprocessing if and only if the spectrum $S_A(\beta )$ has no accumulation point.