Minimal doubling for small subsets in compact Lie groups

Abstract

We prove an asymptotically sharp bound for the minimal doubling of a small measurable subset of a compact connected Lie group, settling a conjecture of Breuillard and Green. Namely, let $G$ be a compact connected Lie group of positive dimension $d_G$ with Haar probability measure ${\mu }_G$, and let $\epsilon >0$. We show that for all compact subsets $A$ of sufficiently small measure we have \[μ_G(A^2) \geq \left(2^{d_G-d_M} - ε\right)μ_G(A),\] where $d_M$ is the maximal dimension of a proper closed subgroup. This recovers, by a different method, the small-measure doubling theorem of Jing--Tran--Zhang for $G={\mathrm{SO}}_3(\mathbb{R})$. More generally, for every $\epsilon >0$ and any pair of non-empty compact subsets $A,B$ of sufficiently small measure, we prove the Brunn--Minkowski-type inequality: \[ μ_G(AB)^{\frac{1}{d_G-d_M}} \geq (1-ε)\left( μ_G(A)^{\frac{1}{d_G-d_M}} + μ_G(B)^{\frac{1}{d_G-d_M}}\right).\] We also prove an inverse theorem in the semi-simple case: for any fixed $K\ge 1$ and $\delta >0$, every compact $K$-approximate subgroup of sufficiently small positive Haar measure is covered by $C(d_G,K)$ translates of the $\delta$-neighbourhood of a proper closed connected subgroup. Our approach combines this inverse theorem with optimal transport and its recent applications to the Brunn--Minkowski inequality.

arXiv comment39 pages. Revised exposition following anonymous comments. The stability result is moved to a separate paper