A problem of Rankin on sets without geometric progressions

TL;DR

This paper constructs maximal sets within (0,1] that avoid geometric progressions of length k with integer ratio, providing new bounds for related combinatorial problems.

Contribution

It introduces a greedy algorithm to build maximal sets avoiding geometric progressions of a given length and ratio, and establishes their structure and bounds.

Findings

Constructed maximal sets avoiding geometric progressions of length k

Proved the sets are unions of intervals defined by a decreasing sequence

Provided new lower bounds for the size of progression-free subsets of integers

Abstract

A geometric progression of length $k$ and integer ratio is a set of numbers of the form $\{a,ar,\dots,ar^{k-1}\}$ for some positive real number $a$ and integer $r\geq 2$. For each integer $k \geq 3$, a greedy algorithm is used to construct a strictly decreasing sequence $(a_i)_{i=1}^{\infty}$ of positive real numbers with $a_1 = 1$ such that the set \[ G^{(k)} = \bigcup_{i=1}^{\infty} \left(a_{2i} , a_{2i-1} \right] \] contains no geometric progression of length $k$ and integer ratio. Moreover, $G^{(k)}$ is a maximal subset of $(0,1]$ that contains no geometric progression of length $k$ and integer ratio. It is also proved that there is a strictly increasing sequence $(A_i)_{i=1}^{\infty}$ of positive integers with $A_1 = 1$ such that $a_i = 1/A_i$ for all $i = 1,2,3,\ldots$. The set $G^{(k)}$ gives a new lower bound for the maximum cardinality of a subset of the set of integers $\{1,2,\dots,n\}$ that contains no geometric progression of length $k$ and integer ratio.