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arXiv math/0007198math.DG
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Milnor球的曲率和对称性

Curvature and symmetry of Milnor spheres

Karsten Grove, Wolfgang Ziller

摘要:

本文研究了上齐性单流形的几何和拓扑,即具有主轨道为超曲面的群作用的流形。我们证明了S^4上每个主SO(3)和SO(4)丛的主群作用都扩张为上齐性1作用。作为结果,我们证明了S^4上的每个向量丛和每个球面丛都有一个非负曲率的完备度量。众所周知,在7维的27个奇异球中,有15个可以用无穷多种方法写成S^4上的S^3丛,因此我们得到了这些奇异球上无穷多个非负曲度规。进一步的推论是SO(3)在S^7上有无穷多个几乎自由作用,即所有的各向同性群都是有限的。这些作用保持了霍普夫纤维化S^3 -> S^7 -> S^4,但没有扩展到圆盘D^8。我们还在上述15个奇异的7-球面上构造了无穷多个这样的作用。

英文摘要:

In this paper we explore the geometry and topology of cohomogeneity one manifolds, i.e. manifolds with a group action whose principal orbits are hypersurfaces. We show that the principal group action of every principal SO(3) and SO(4) bundle over S^4 extends to a cohomogeneity one action. As a consequence we prove that every vector bundle and every sphere bundle over S^4 has a complete metric with non-negative curvature. It is well known that 15 of the 27 exotic spheres in dimension 7 can be written as S^3 bundles over S^4 in infinitely many ways, and hence we obtain infinitely many non-negatively curved metrics on these exotic spheres. A further consequence will be that there are infinitely many almost free actions by SO(3) on S^7, i.e. all isotropy groups are finite. These actions preserve the Hopf fibration S^3 -> S^7 -> S^4 but do not extend to the disc D^8. We also construct infinitely many such actions on the 15 exotic 7-spheres mentioned above.

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