Group actions and a multi-parameter Falconer distance problem

TL;DR

This paper investigates a multi-parameter version of the Falconer distance problem, establishing conditions on the Hausdorff dimension of sets in Euclidean space that guarantee positive Lebesgue measure of the multi-parameter distance set.

Contribution

The paper introduces a new multi-parameter Falconer distance problem and proves a dimension threshold for the positivity of the measure, utilizing a group action approach and a Mattila integral.

Findings

If each dimension d_i ≥ 2, then a set with Hausdorff dimension greater than d - (min d_i)/2 + 1/3 ensures positive measure.

The result improves understanding of how multi-parameter distances relate to set dimensions.

The conclusion does not hold if the set's dimension is below d - (min d_i)/2, as shown by previous constructions.

Abstract

In this paper we study the following multi-parameter variant of the celebrated Falconer distance problem. Given ${\textbf{d}}=(d_1,d_2, \dots, d_{\ell})\in \mathbb{N}^{\ell}$ with $d_1+d_2+\dots+d_{\ell}=d$ and $E \subseteq \mathbb{R}^d$, we define $$ \Delta_{{\textbf{d}}}(E) = \left\{ \left(|x^{(1)}-y^{(1)}|,\ldots,|x^{(\ell)}-y^{(\ell)}|\right) : x,y \in E \right\} \subseteq \mathbb{R}^{\ell}, $$ where for $x\in \mathbb{R}^d$ we write $x=\left( x^{(1)},\dots, x^{(\ell)} \right)$ with $x^{(i)} \in \mathbb{R}^{d_i}$. We ask how large does the Hausdorff dimension of $E$ need to be to ensure that the $\ell$-dimensional Lebesgue measure of $\Delta_{{\textbf{d}}}(E)$ is positive? We prove that if $2 \leq d_i$ for $1 \leq i \leq \ell$, then the conclusion holds provided $$ \dim(E)>d-\frac{\min d_i}{2}+\frac{1}{3}.$$ We also note that, by previous constructions, the conclusion does not in general hold if $$\dim(E)<d-\frac{\min d_i}{2}.$$ A group action derivation of a suitable Mattila integral plays an important role in the argument.