Universal $\beta$-expansions

Abstract

Given $\beta\in(1,2)$, a $\beta$-expansion of a real $x$ is a power series in base $\beta$ with coefficients 0 and 1 whose sum equals $x$. The aim of this note is to study certain problems related to the universality and combinatorics of $\beta$-expansions. Our main result is that for any $\beta\in(1,2)$ and a.e. $x\in (0,1)$ there always exists a universal $\beta$-expansion of $x$ in the sense of Erd\"os and Komornik, i.e., a $\beta$-expansion whose complexity function is $2^n$. We also study some questions related to the points having less than a full branching continuum of $\beta$-expansions and also normal $\beta$-expansions.