Sumsets of Ahlfors--David regular sets

Abstract

We prove that there is an absolute constant $c>0$ such that for every finite $(\alpha ,M)$-Ahlfors--David regular set $A\subseteq [N]$ with $0\le \alpha <1$ and $M\ge 2$, we have the sumset estimate \[|A+A|\geq |A|^{1+c(1-α)/\log M}.\] We also prove a corresponding statement for Ahlfors--David regular sets in $[0,1]$. The proof combines a multiscale entropy decomposition with inverse results from additive combinatorics, showing that Ahlfors--David regularity forces a definite entropy gain at each scale. The dependence $1/\log M$ in the exponent is of optimal order.

19 pages