The Quasi Curvature-Dimension Condition with applications to sub-Riemannian manifolds

Abstract: We obtain the best known quantitative estimates for the $L^p$-Poincar\'e and log-Sobolev inequalities on domains in various sub-Riemannian manifolds, including ideal Carnot groups and in particular ideal generalized H-type Carnot groups and the Heisenberg groups, corank $1$ Carnot groups, the Grushin plane, and various H-type foliations, Sasakian and $3$-Sasakian manifolds. Moreover, this constitutes the first time that a quantitative estimate independent of the dimension is established on these spaces. For instance, the Li-Yau / Zhong-Yang spectral-gap estimate holds on all Heisenberg groups of arbitrary dimension up to a factor of $4$. We achieve this by introducing a quasi-convex relaxation of the Lott-Sturm-Villani $\mathsf{CD}(K,N)$ condition we call the Quasi Curvature-Dimension condition $\mathsf{QCD}(Q,K,N)$. Our motivation stems from a recent interpolation inequality along Wasserstein geodesics in the ideal sub-Riemannian setting due to Barilari and Rizzi. We show that on an ideal sub-Riemannian manifold of dimension $n$, the Measure Contraction Property $\mathsf{MCP}(K,N)$ implies $\mathsf{QCD}(Q,K,N)$ with $Q = 2^{N-n} \geq 1$, thereby verifying the latter property on the aforementioned ideal spaces; a result of Balogh-Krist\'aly-Sipos is used instead to handle non-ideal corank $1$ Carnot groups. By extending the localization paradigm to completely general interpolation inequalities, we reduce the study of various analytic and geometric inequalities on $\mathsf{QCD}$ spaces to the one-dimensional case. Consequently, we deduce that while (strictly) sub-Riemannian manifolds do not satisfy any type of $\mathsf{CD}$ condition, many of them satisfy numerous functional inequalities with \emph{exactly the same} quantitative dependence (up to a factor of $Q$) as their $\mathsf{CD}$ counterparts.