Title: Group actions and a multi-parameter Falconer distance problem,
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.