$\mathfrak{B}$-free integers in number fields and dynamics

Abstract

In 2010, Sarnak initiated the study of the dynamics of the system determined by the square of the Möbius function (the characteristic function of the square-free integers). We deal with his program in the more general context of $\mathfrak{B}$-free integers in number fields, suggested 5 years later by Baake and Huck. This setting encompasses the classical square-free case and its generalizations. Given a number field $K$, let $\mathfrak{B}$ be a family of pairwise coprime ideals in its ring of integers ${\mathcal{O}}_K$, such that ${\sum }_{\mathfrak{b}\in \mathfrak{B}}1/|{\mathcal{O}}_K/\mathfrak{b}|<\infty$. We study the dynamical system determined by the set ${\mathcal{F}}_{\mathfrak{B}}={\mathcal{O}}_K\setminus {\bigcup }_{\mathfrak{b}\in \mathfrak{B}}\mathfrak{b}$ of $\mathfrak{B}$-free integers in ${\mathcal{O}}_K$. We show that the characteristic function $1_{{\mathcal{F}}_{\mathfrak{B}}}$ of ${\mathcal{F}}_{\mathfrak{B}}$ is generic along the natural Følner sequence for a probability measure on $\{0,1{\}}^{{\mathcal{O}}_K}$, invariant under the multidimensional shift. The corresponding measure-theoretical dynamical system is proved to be isomorphic to an ergodic rotation on a compact Abelian group. In particular, it is of zero Kolmogorov entropy. Moreover, we provide a description of ``patterns'' appearing in ${\mathcal{F}}_{\mathfrak{B}}$ and compute the topological entropy of the orbit closure of $1_{{\mathcal{F}}_{\mathfrak{B}}}$. Finally, we show that this topological dynamical system has a non-trivial topological joining with an ergodic rotation on a compact Abelian group.