A nonabelian Brunn–Minkowski inequality

Journal article

We prove a sharp bound for the minimal doubling of a small measurable subset of a compact connected Lie group. Namely, let $G$ be a compact connected Lie group of dimension $d_G$, we show that for for all measurable subsets $A$, we have $$\mu_G(A^2) \geq \left(2^{d_G-d_H} - C\mu_G(A)^{\frac{2}{d_G-d_H}}\right)\mu_G(A)$$ where $d_H$ is the maximal dimension of a proper closed subgroup $H$ and $C > 0$ is a dimensional constant. This settles a conjecture of Breuillard and Green, and recovers and improves - with completely different methods - a recent result of Jing--Tran--Zhang corresponding to the case $G=SO_3(\mathbb{R})$. As is often the case, the above doubling inequality stems from a special case of general product-set estimates. We prove that for all $\epsilon >0$ and for any pair of sufficiently small measurable subsets $A,B$ a Brunn--Minkowski-type inequality holds: $$ \mu_G(AB)^{\frac{1}{d_G-d_H}} \geq (1-\epsilon)\left( \mu_G(A)^{\frac{1}{d_G-d_H}} + \mu_G(B)^{\frac{1}{d_G-d_H}}\right).$$ Going beyond the scope of the Breuillard--Green conjecture, we prove a stability result asserting that the only subsets with close to minimal doubling are essentially neighbourhoods of proper subgroups i.e. of the form $$H_{\delta}:=\{g \in G: d(g,H)<\delta\}$$ where $H$ denotes a proper closed subgroup of maximal dimension, $d$ denotes a bi-invariant distance on $G$ and $\delta >0$. Our approach relies on a combination of two toolsets: optimal transports and its recent applications to the Brunn--Minkowski inequality, and the structure theory of compact approximate subgroups.Comment: 44 pages. New proof of Proposition 1.6 relies on discretization/multi-scale analysis of approximate groups, rather than on an unpublished result due to Carolino. The proof is now, in principle, quantitative. Submitte

Authors: Jing, Y; Tran, C-M; Zhang, R

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Springer
• Journal: Geometric And Functional Analysis
• Volume: 33
• Issue: 4
• Pages: 1048-1100
• Publication date: 2023-07-05
• DOI: 10.1007/s00039-023-00647-6
• EISSN: 1420-8970
• ISSN: 1016-443X
• Language: English
• Keywords: Log sum inequality; Geometry; Combinatorics; Inequality; Algebraic number; Mathematical analysis; Mathematics; Minkowski inequality; Lie group; Pure mathematics; Minkowski space; Rearrangement inequality; Locally compact space; Algebra over a field; Center (category theory)