Similar point configurations via group actions

TL;DR

This paper proves that sets with sufficiently large Hausdorff dimension in Euclidean space contain similar point configurations, extending previous results with weaker dimensional thresholds and applying group actions.

Contribution

It introduces a new approach to find similar point configurations in thin sets using group actions, improving upon earlier dimensional constraints.

Findings

Existence of similar k-simplices in sets with Hausdorff dimension > d^2/(2d-1)

Extension of similarity results to arbitrary continuous maps

Application of group-theoretic methods in finite fields

Abstract

We prove that for $d\ge 2,\, k\ge 2$, if the Hausdorff dimension of a compact set $E\subset \mathbb{R}^d$ is greater than $\frac{d^2}{2d-1}$, then, for any given $r > 0$, there exist $(x^1, \dots, x^{k+1})\in E^{k+1}$, $(y^1, \dots, y^{k+1})\in E^{k+1}$, a rotation $\theta \in \mathrm{O}_d(\mathbb{R})$, and a vector $a \in \mathbb{R}^d$ such that $rx^j = \theta y^j - a$ for $1 \leq j \leq k+1$. Such a result on existence of similar $k$-simplices in thin sets had previously been established under a more stringent dimensional threshold in Greenleaf, Iosevich and Mkrtchyan \cite{GIM21}. The argument we are use to prove the main result here was previously employed in Bhowmik and Rakhmonov \cite{BR23} to establish a finite field version. We also show the existence of multi-similarities of arbitrary multiplicity in $\R^d$, show how to extend these results from similarities to arbitrary proper continuous maps, as well as explore a general group-theoretic formulation of this problem in vector spaces over finite fields.