Poincar\'e and Brunn--Minkowski inequalities on the boundary of weighted Riemannian manifolds

TL;DR

This paper extends classical inequalities like Poincaré and Brunn-Minkowski to weighted Riemannian manifolds with convex boundaries, introduces a new geometric flow, and establishes novel inequalities and spectral estimates in this setting.

Contribution

It generalizes Euclidean inequalities to weighted Riemannian manifolds, introduces a new geometric evolution equation, and derives new Brunn-Minkowski and spectral-gap inequalities.

Findings

Established Poincaré-type inequalities on boundary under convexity assumptions.

Derived a new Brunn-Minkowski inequality for weighted Riemannian domains.

Proposed a novel geometric flow related to Monge-Ampère equations.

Abstract

We study a Riemannian manifold equipped with a density which satisfies the Bakry--\'Emery Curvature-Dimension condition (combining a lower bound on its generalized Ricci curvature and an upper bound on its generalized dimension). We first obtain a Poincar\'e-type inequality on its boundary assuming that the latter is locally-convex; this generalizes a purely Euclidean inequality of Colesanti, originally derived as an infinitesimal form of the Brunn-Minkowski inequality, thereby precluding any extensions beyond the Euclidean setting. A dual version for generalized mean-convex boundaries is also obtained, yielding spectral-gap estimates for the weighted Laplacian on the boundary. Motivated by these inequalities, a new geometric evolution equation is proposed, which extends to the Riemannian setting the Minkowski addition operation of convex domains, a notion thus far confined to the purely linear setting. This geometric flow is characterized by having parallel normals (of varying velocity) to the evolving hypersurface along the trajectory, and is intimately related to a homogeneous Monge-Amp\`ere equation on the exterior of the convex domain. Using the aforementioned Poincar\'e-type inequality on the boundary of the evolving hypersurface, we obtain a novel Brunn--Minkowski inequality in the weighted-Riemannian setting, amounting to a certain concavity property for the weighted-volume of the evolving enclosed domain. All of these results appear to be new even in the classical non-weighted Riemannian setting.