The Thickness of Infinite Sidon Sets

Abstract

Let $γ\ge 1$. A set $A$ of nonnegative integers is a Sidon set if for each $d>0$ there is at most one pair $(a,b) \in A \times A$ with $d=a-b$. If there are at most $γ$ pairs, then $A$ is a $γ$-Golomb ruler. We prove that if $A$ is a $γ$-Golomb ruler, then \[\liminf_{n\to\infty} \frac{|A\cap[0,n)|}{\sqrt{n/\log n}} \le \frac{2}{\sqrt{\log 2}} \sqrtγ.\] There is a $γ$-Golomb ruler $G$ with \[ \limsup_{n\to\infty} \frac{|G\cap[0,n)|}{\sqrt n} \ge \frac{1}{\sqrt2} \sqrtγ.\]