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Centro-affine hypersurface immersions with parallel cubic form

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Abstract

We consider non-degenerate centro-affine hypersurface immersions in \(\mathbb R^n\) whose cubic form is parallel with respect to the Levi-Civita connection of the affine metric. There exists a bijective correspondence between homothetic families of proper affine hyperspheres with center in the origin and with parallel cubic form, and Köchers conic \(\omega \)-domains, which are the maximal connected sets consisting of invertible elements in a real semi-simple Jordan algebra. Every level surface of the \(\omega \) function in an \(\omega \)-domain is an affine complete, Euclidean complete proper affine hypersphere with parallel cubic form and with center in the origin. On the other hand, every proper affine hypersphere with parallel cubic form and with center in the origin can be represented as such a level surface. We provide a complete classification of proper affine hyperspheres with parallel cubic form based on the classification of semi-simple real Jordan algebras. Centro-affine hypersurface immersions with parallel cubic form are related to the wider class of real unital Jordan algebras. Every such immersion can be extended to an affine complete one, whose conic hull is the connected component of the unit element in the set of invertible elements in a real unital Jordan algebra. Our approach can be used to study also other classes of hypersurfaces with parallel cubic form.

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Notes

  1. The claim in Köcher (1999, item (v), pp. 71–72) that \(J\) must be semi-simple is false. A counter-example will be given in Sect. 5.6.1.

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Acknowledgments

The author would like to thank Professors Zejun Hu, An-Min Li, Udo Simon, and Luc Vrancken for enlightening discussions on the subject.

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  1. WIAS, Mohrenstrasse 39, 10117, Berlin, Germany

    Roland Hildebrand

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  1. Roland Hildebrand
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Hildebrand, R. Centro-affine hypersurface immersions with parallel cubic form. Beitr Algebra Geom 56, 593–640 (2015). https://doi.org/10.1007/s13366-014-0216-4

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