On density of infinite subsets I

Abstract

Let $Y$ be a compact metric space, $G$ be a group acting by transformations on $Y$. For any infinite subset $A\subset Y$, we study the density of $gA$ for $g\in G$ and quantitative density of the set $\bigcup _{g\in G_n}gA$ by the Hausdorff semimetric $d^H$. It is proven that for any integer $n\ge 2$, $\epsilon >0$, any infinite subset $A\subset {\mathbb{T}}^n$, there is a $g\in SL(n,\mathbb{Z})$ such that $gA$ is $\epsilon$-dense. We also show that, for any infinite subset $A\subset [0,1]$, for generic rotation and generic 3-IET, $$\underset{n}{\mathrm{liminf}}n\cdot d^H\left(\bigcup _{k=0}^{n-1}{T}^{k}A,[0,1]\right)=0.$$