A problem of Rankin on sets without geometric progressions (1408.2880v1)

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.