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arXiv 1001.4569math.DG
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共形平坦流形的对偶性

The duality of conformally flat manifolds

Huili Liu, Masaaki Umehara, Kotaro Yamada

摘要:

在与Saji的合作中,第二和第三作者给出了波前的内在公式,并证明了波前在空间型中的实现定理。作为应用,我们证明了以下四个对象本质上是相同的:* 具有容许奇点的共形平坦n-流形(n ≥ 3)(即容许GCF-流形),* 作为光锥Q^{n+1}_+中的超曲面的锋,* 作为双曲空间H^{n+1}中的超曲面的锋,* 作为de Sitter空间S^{n+1}_1中的超曲面的sapacelike锋.近年来,一些几何学家发现了共形平坦黎曼流形的对偶性。在我们的设置中,这种对偶性可以通过容许GCF-流形的同余类到H^{n+1}中的frontal的二重映射的存在性来解释。值得注意的是,即使原共形平坦度量是正定的,对偶共形平坦度量也可能存在退化点。这就是为什么我们考虑具有奇点的共形平坦流形的原因。实际上,对偶是可容许GCF-流形集合上的对合。n=2的情形需要特殊处理,因为任何黎曼2-流形都是共形平坦的。最后,我们还确定了单连通2维黎曼流形到光锥Q^{3}+的等距浸入的模空间.

英文摘要:

In a joint work with Saji, the second and the third authors gave an intrinsic formulation of wave fronts and proved a realization theorem of wave fronts in space forms. As an application, we show that the following four objects are essentially same; * conformally flat n-manifolds (n>=3) with admissible singular points (i.e. admissible GCF-manifolds), * frontals as hypersurfaces in the lightcone Q^{n+1}_+, * frontals as hypersurfaces in the hyperbolic space H^{n+1}, * sapacelike frontals as hypersurfaces in the de Sitter space S^{n+1}_1. Recently, the duality of conformally flat Riemannian manifolds was found by several geometers. In our setting, this duality can be explained via the existence of a two-fold map of the congruent classes of admissible GCF-manifolds into that of frontals in H^{n+1}. It should be remarked that the dual conformally flat metric may have degenerate points even when the original conformally flat metric is positive definite. This is the reason why we consider conformally flat manifolds with singular points. In fact, the duality is an involution on the set of admissible GCF-manifolds. The case n=2 requires an exceptional treatment, since any Riemannian 2-manifolds are conformally flat. At the end of this paper, we also determine the moduli space of isometric immersions of a given simply connected Riemannian 2-manifold into the lightcone Q^{3}_+.

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