On sets with small sumset in the circle

Abstract

We prove results on the structure of a subset of the circle group having positive inner Haar measure and doubling constant close to the minimum. These results go toward a continuous analogue in the circle of Freiman's $3k-4$ theorem from the integer setting. An analogue of this theorem in $\mathbb{Z}_p$ has been pursued extensively, and we use some recent results in this direction. For instance, obtaining a continuous analogue of a result of Serra and Z\'emor, we prove that if a subset $A$ of the circle is not too large and has doubling constant at most $2+\varepsilon$ with $\varepsilon<10^{-4}$, then for some integer $n>0$ the dilate $n\cdot A$ is included in an interval in which it has density at least $1/(1+\varepsilon)$. Our arguments yield other variants of this result as well, notably a version for two sets which makes progress toward a conjecture of Bilu. We include two applications of these results. The first is a new upper bound on the size of $k$-sum-free sets in the circle and in $\mathbb{Z}_p$. The second gives structural information on subsets of $\mathbb{R}$ of doubling constant at most $3+\varepsilon$.