Hitting times of shrinking targets: Transversality and an ergodic theorem

Abstract

In this paper, we investigate ergodic and fractal properties of the sets $${\Lambda }_y:=\{n\in \mathbb{N}:\{u_{n}y\}\in I_n\},$$ where $\{\cdot \}$ denotes the fractional part function, $(u_n{)}_{n\in \mathbb{N}}$ is an increasing sequence of real numbers, $y\in [0,1]$ and each $I_n$ is a finite union of intervals with decreasing Lebesgue measure. Our main result shows that, under suitable conditions, the set ${\Lambda }_y$ is good for pointwise convergence of ergodic averages for Lebesgue almost every $y\in [0,1]$. Furthermore, we prove a transversality phenomenon: for any fixed set $A\subseteq \mathbb{N}$, the sets ${\Lambda }_y$ and $A$ are geometrically independent for almost every $y\in [0,1]$, as witnessed by the integer-fractal dimension of their intersection

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