Abstract
For Riemannian submanifolds of a semi-Riemannian manifold, we introduce the concepts of total shear tensor and shear operators as the trace-free part of the corresponding second fundamental form and shape operators. The relationship between these quantities and the umbilical properties of the submanifold is shown. Several novel notions of umbilical submanifolds are then considered along with the classical concepts of totally umbilical and pseudo-umbilical submanifolds. Then we focus on the case of co-dimension 2, and we present necessary and sufficient conditions for the submanifold to be umbilical with respect to a normal direction. Moreover, we prove that the umbilical direction, if it exists, is unique —unless the submanifold is totally umbilical— and we give a formula to compute it explicitly. When the ambient manifold is Lorentzian we also provide a way of determining its causal character. We end the paper by illustrating our results on the Lorentzian geometry of the Kerr black hole.
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This work is partially supported by the Belgian Interuniversity Attraction Pole P07/18 (Dygest), by the KU Leuven Research Fund project 3E160361 “Lagrangian and calibrated submanifolds”, and was initiated during a visit of the first author to the UPV/EHU supported by a travel grant of the Research Foundation–Flanders (FWO). NC and JMMS are supported under grant FIS2014-57956-P (Spanish MINECO–Fondos FEDER) and project IT956-16 of the Basque Government. JMMS is also supported under project UFI 11/55 (UPV/EHU).
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Cipriani, N., Senovilla, J.M.M. & Van der Veken, J. Umbilical Properties of Spacelike Co-dimension Two Submanifolds. Results Math 72, 25–46 (2017). https://doi.org/10.1007/s00025-016-0640-x
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DOI: https://doi.org/10.1007/s00025-016-0640-x