## Osaka Journal of Mathematics

### A new look at Condition A

Quo-Shin Chi

#### Abstract

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.

#### Article information

Source
Osaka J. Math., Volume 49, Number 1 (2012), 133-166.

Dates
First available in Project Euclid: 21 March 2012

Permanent link to this document
https://projecteuclid.org/euclid.ojm/1332337242

Mathematical Reviews number (MathSciNet)
MR2903258

Zentralblatt MATH identifier
1246.53078

Subjects
Primary: 53C40: Global submanifolds [See also 53B25]

#### Citation

Chi, Quo-Shin. A new look at Condition A. Osaka J. Math. 49 (2012), no. 1, 133--166. https://projecteuclid.org/euclid.ojm/1332337242

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