Dynamical behavior of alternate base expansions

Émilie Charlier, Célia Cisternino, Karma Dajani

Published 2021-02-17Version 1

We generalize the greedy and lazy $\beta$-transformations for a real base $\beta$ to the setting of alternate bases $\boldsymbol{\beta}=(\beta_0,\ldots,\beta_{p-1})$, which were recently introduced by the first and second authors as a particular case of Cantor bases. As in the real base case, these new transformations, denoted $T_\boldsymbol{\beta}$ and $L_\boldsymbol{\beta}$ respectively, can be iterated in order to generate the digits of the greedy and lazy $\boldsymbol{\beta}$-expansions of real numbers. The aim of this paper is to describe the dynamical behaviors of $T_\boldsymbol{\beta}$ and $L_\boldsymbol{\beta}$. We first prove the existence of a unique absolutely continuous (with respect to an extended Lebesgue measure, called the $p$-Lebesgue measure) $T_\boldsymbol{\beta}$-invariant measure. We then show that this unique measure is in fact equivalent to the $p$-Lebesgue measure and that the corresponding dynamical system is ergodic and has entropy $\frac{1}{p}\log(\beta_{p-1}\cdots \beta_0)$. We then express the density of this measure and compute the frequencies of letters in the greedy $\boldsymbol{\beta}$-expansions. We obtain the dynamical properties of $L_\boldsymbol{\beta}$ by showing that the lazy dynamical system is isomorphic to the greedy one. We also provide an isomorphism with a suitable extension of the $\beta$-shift. Finally, we show that the $\boldsymbol{\beta}$-expansions can be seen as $(\beta_{p-1}\cdots \beta_0)$-representations over general digit sets and we compare both frameworks.