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SUB-RIEMANNIAN RICCI CURVATURES AND UNIVERSAL DIAMETER BOUNDS FOR 3-SASAKIAN MANIFOLDS

Published online by Cambridge University Press:  21 June 2017

Luca Rizzi
Affiliation:
University of Grenoble Alpes, CNRS, Institut Fourier, F-38000 Grenoble, France Inria, team GECO & CMAP, École Polytechnique, CNRS, Université Paris-Saclay, Palaiseau, France (past institution) (luca.rizzi@univ-grenoble-alpes.fr)
Pavel Silveira
Affiliation:
Leibniz Universität Hannover, Institut für Analysis, Germany (psilveir@math.uni-hannover.de)

Abstract

For a fat sub-Riemannian structure, we introduce three canonical Ricci curvatures in the sense of Agrachev–Zelenko–Li. Under appropriate bounds we prove comparison theorems for conjugate lengths, Bonnet–Myers type results and Laplacian comparison theorems for the intrinsic sub-Laplacian. As an application, we consider the sub-Riemannian structure of 3-Sasakian manifolds, for which we provide explicit curvature formulas. We prove that any complete 3-Sasakian structure of dimension $4d+3$, with $d>1$, has sub-Riemannian diameter bounded by $\unicode[STIX]{x1D70B}$. When $d=1$, a similar statement holds under additional Ricci bounds. These results are sharp for the natural sub-Riemannian structure on $\mathbb{S}^{4d+3}$ of the quaternionic Hopf fibrations:

$$\begin{eqnarray}\mathbb{S}^{3}{\hookrightarrow}\mathbb{S}^{4d+3}\rightarrow \mathbb{HP}^{d},\end{eqnarray}$$
whose exact sub-Riemannian diameter is $\unicode[STIX]{x1D70B}$, for all $d\geqslant 1$.

Type
Research Article
Copyright
© Cambridge University Press 2017 

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