A problem of Rankin on sets without geometric progressions

Abstract

A geometric progression of length $k$ and integer ratio is a set of numbers of the form $\{a,ar,\dots ,a{r}^{k-1}\}$ for some positive real number $a$ and integer $r\ge 2$. For each integer $k\ge 3$, 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 $i=1,2,3,\dots$. 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.

15 pages