Uniform Diophantine approximation related to $b$-ary and $\beta$-expansions

25 pagesInternational audienceLet $b\geq 2$ be an integer and $\hv$ a real number. Among other results, we compute the Hausdorff dimension of the set of real numbers $\xi$ with the property that, for every sufficiently large integer $N$, there exists an integer $n$ such that $1 \le n \le N$ and the distance between $b^n \xi$ and its nearest integer is at most equal to $b^{-\hv N}$. We further solve the same question when replacing $b^n\xi$ by $T^n_\beta \xi$, where $T_\beta$ denotes the classical $\beta$-transformation