The Hausdorff dimension of the intersection of $ψ$-well approximable numbers and self-similar sets

Abstract

Let $ψ:\mathbb{N}\rightarrow\mathbb{R}_+$ be a monotonically non-increasing function, and let $ψ_v:\mathbb{N}\rightarrow\mathbb{R}_+$ be defined by $ψ_v(q)=1/q^v$. In this article, we consider self-similar sets whose iterated function systems satisfy the open set condition. For functions $ψ$ that do not decrease too rapidly, we give a conjecturally sharp upper bound on the Hausdorff dimension of the intersection of $ψ$-well approximable numbers and such self-similar sets. When $ψ=ψ_v$ for some $v$ greater than 1 and sufficiently close to $1$, we give a lower bound for this Hausdorff dimension, which asymptotically matches the upper bound as $v\downarrow 1$. In particular, we show that the set of very well approximable numbers has full Hausdorff dimension within self-similar sets, thus confirming a conjecture of Levesley, Salp, and Velani.