Large sets avoiding affine copies of infinite sequences

TL;DR

This paper constructs measure-zero sets of Hausdorff dimension one that avoid all nontrivial affine copies of certain infinite decreasing sequences converging to zero, addressing a special case of Erdős's conjecture.

Contribution

It introduces a method to build measure-zero, Hausdorff dimension one sets avoiding affine copies of sequences with prescribed decay rates, including geometric sequences.

Findings

Constructed sets of Hausdorff dimension 1 with Lebesgue measure zero

Sets avoid all affine copies of specified decreasing sequences

Applicable to sequences with polynomial decay rates

Abstract

A conjecture of Erd\H{o}s states that for any infinite set $A \subseteq \mathbb R$, there exists $E \subseteq \mathbb R$ of positive Lebesgue measure that does not contain any nontrivial affine copy of $A$. The conjecture remains open for most fast-decaying sequences, including the geometric sequence $A = \{2^{-k} : k \geq 1\}$. In this article, we consider infinite decreasing sequences $A = \{a_k: k \geq 1\}$ in ${\mathbb R}$ that converge to zero at a prescribed rate; namely $\log (a_n/a_{n+1}) = e^{\varphi(n)} $, where $\varphi(n)/n\to 0$ as $n\to\infty$. This condition is satisfied by sequences whose logarithm has polynomial decay, and in particular by the geometric sequence. For any such sequence $A$, we construct a Borel set ${\mathcal O}\subseteq \mathbb R$ of Hausdorff dimension 1, but Lebesgue measure zero, that avoids all nontrivial affine copies of $A\cup\{0\}$.