Beiträge zur Algebra und Geometrie Contributions to Algebra and Geometry Vol. 45, No. 2, pp. 697702 (2004) 

A characterization of isoparametric hypersurfaces of Clifford typeStefan ImmervollMathematisches Institut, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany, email: stim@fa.unituebingen.deAbstract: Let $M$ be an isoparametric hypersurface with four distinct principal curvatures in the unit sphere $\bf S \subseteq {\bf R}^{2l}$ with focal manifolds $M_+$ and $M_$. Let $\cal U$ be a vector space of symmetric $(2l\times 2l)$matrices such that each matrix in $\cal U\backslash \set{0}$ is regular, and assume that $M_+$ is the intersection of $\bf S$ with the quadrics ${x \in \bf R^{2l}}{\mid {x,Ax}=0}$, $A\in \cal U$. Then $\cal U$ is generated by a Clifford system and $M$ is an isoparametric hypersurface of Clifford type provided that $\dim M_+ \geq \dim M_$. The proof of this theorem is based on properties of quadratic forms vanishing on $M_+$ and on a structure theorem for isoparametric triple systems, which we prove in this paper. Keywords: isoparametric hypersurface, triple system, Peirce decomposition, Clifford system/sphere Classification (MSC2000): 53C40; 17A40, 15A66 Full text of the article:
Electronic version published on: 9 Sep 2004. This page was last modified: 4 May 2006.
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