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March 2012 A new look at Condition A
Quo-Shin Chi
Osaka J. Math. 49(1): 133-166 (March 2012).


Ozeki and Takeuchi [14] introduced the notion of Condition A and Condition B to construct two classes of inhomogeneous isoparametric hypersurfaces with four principal curvatures in spheres, which were later generalized by Ferus, Karcher and Münzner to many more examples via the Clifford representations; we will refer to these examples of Ozeki and Takeuchi and of Ferus, Karcher and Münzner collectively as OT-FKM type throughout the paper. Dorfmeister and Neher [5] then employed isoparametric triple systems [3, 4], which are algebraic in nature, to prove that Condition A alone implies the isoparametric hypersurface is of OT-FKM type. Their proof for the case of multiplicity pairs $\{3, 4\}$ and $\{7, 8\}$ rests on a fairly involved algebraic classification result [9] about composition triples. In light of the classification [2] that leaves only the four exceptional multiplicity pairs $\{4, 5\}, \{3, 4\}, \{7, 8\}$ and $\{6, 9\}$ unsettled, it appears that Condition A may hold the key to the classification when the multiplicity pairs are $\{3, 4\}$ and $\{7, 8\}$. Thus Condition A deserves to be scrutinized and understood more thoroughly from different angles. In this paper, we give a fairly short and rather straightforward proof of the result of Dorfmeister and Neher, with emphasis on the multiplicity pairs $\{3, 4\}$ and $\{7, 8\}$, based on more geometric considerations. We make it explicit and apparent that the octonion algebra governs the underlying isoparametric structure.


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Quo-Shin Chi. "A new look at Condition A." Osaka J. Math. 49 (1) 133 - 166, March 2012.


Published: March 2012
First available in Project Euclid: 21 March 2012

zbMATH: 1246.53078
MathSciNet: MR2903258

Primary: 53C40

Rights: Copyright © 2012 Osaka University and Osaka City University, Departments of Mathematics

Vol.49 • No. 1 • March 2012
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