Sub-Laplacian comparison theorems on totally geodesic Riemannian foliations
by Fabrice Baudoin, Erlend Grong, Kazumasa Kuwada and Anton Thalmaier
We develop a variational theory of geodesics for the canonical variation of the metric of a totally geodesic foliation. As a consequence, we obtain comparison theorems for the horizontal and vertical Laplacians. In the case of Sasakian foliations, we show that sharp horizontal and vertical comparison theorems for the sub-Riemannian distance may be obtained as a limit of horizontal and vertical comparison theorems for the Riemannian distances approximations.
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