Sums, Differences and Dilates

Abstract

Given a set of integers $A$ and an integer $k$, write $A+k\cdot A$ for the set $\{a+kb:a\in A,b\in A\}$. Hanson and Petridis showed that if $|A+A|\le K|A|$ then $|A+2\cdot A|\le K^{2.95}|A|$. At a presentation of this result, Petridis stated that the highest known value for $\frac{\log(|A+2\cdot A|/|A|)}{\log(|A+A|/|A|)}$ (bounded above by 2.95) was $\frac{\log 4}{\log 3}$. We show that, for all $\epsilon>0$, there exist $A$ and $K$ with $|A+A|\le K|A|$ but with $|A+2\cdot A|\ge K^{2-\epsilon}|A|$. Further, we analyse a method of Ruzsa, and generalise it to give continuous analogues of the sizes of sumsets, differences and dilates. We apply this method to a construction of Hennecart, Robert and Yudin to prove that, for all $\epsilon>0$, there exists a set $A$ with $|A-A|\ge |A|^{2-\epsilon}$ but with $|A+A|<|A|^{1.7354+\epsilon}$. The second author would like to thank E. Papavassilopoulos for useful discussions about how to improve the efficiency of his computer searches.