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arXiv 1005.0540math.DGmath.FA
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关于次Riemannian geometry中的Hausdorff体积

On the Hausdorff volume in sub-Riemannian geometry

Andrei Agrachev, Davide Barilari, Ugo Boscain

摘要:

对于正则次黎曼流形,研究了球面Hausdorff测度关于光滑体积的Radon-Nikodym导数.我们证明了在幂零近似下这是单位球的体积,并且它总是一个连续函数。然后,我们证明,直到4维,它是光滑的,而从5维开始,在corank 1的情况下,它是C^3(在每个平滑曲线上都是C^4),但一般不是C^5。这些结果回答了蒙哥马利提出的一个问题,关于两个内在的卷之间的关系,可以定义在一个子黎曼流形,即波普和豪斯多夫量。如果幂零近似依赖于点(这可能从5维开始发生),那么它们一般不成比例。

英文摘要:

For a regular sub-Riemannian manifold we study the Radon-Nikodym derivative of the spherical Hausdorff measure with respect to a smooth volume. We prove that this is the volume of the unit ball in the nilpotent approximation and it is always a continuous function. We then prove that up to dimension 4 it is smooth, while starting from dimension 5, in corank 1 case, it is C^3 (and C^4 on every smooth curve) but in general not C^5. These results answer to a question addressed by Montgomery about the relation between two intrinsic volumes that can be defined in a sub-Riemannian manifold, namely the Popp and the Hausdorff volume. If the nilpotent approximation depends on the point (that may happen starting from dimension 5), then they are not proportional, in general.

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