Dimension of Diophantine approximation and applications

Longhui Li, Bochen Liu

Published 2024-09-19Version 1

In this paper we construct a new family of sets via Diophantine approximation, in which the classical examples are endpoints. Our first application is on their Hausdorff dimension. We show a recent result of Ren and Wang, known sharp on orthogonal projections in the plane, is also sharp on $A+cB$, $c\in C$, thus completely settle this ABC sum-product problem. Higher dimensional examples are also discussed. In addition to Hausdorff dimension, we also consider Fourier dimension. In particular, now for every $0\leq t\leq s\leq 1$ we have an explicit construction in $\mathbb{R}$ of Hausdorff dimension $s$ and Fourier dimension $t$, together with a measure $\mu$ that captures both dimensions. In the end we provide a perspective of Knapp's example and treat our Diophantine approximation as its analog in $\mathbb{R}$, that naturally leads to the sharpness of Mockenhaupt-Mitsis-Bak-Seeger Fourier restriction theorem. These are alternatives of recent examples due to Fraser-Hambrook-Ryou. In particular, to deal with the non-geometric case we construct measures of "Hausdorff dimension" $a$ and Fourier dimension $b$, even if $a<b$. This clarifies some difference between sets and measures.