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arXiv 1109.4641math.FA
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具有Heisenberg目标的索伯列夫空间中Lipschitz映射的缺乏稠密性

On the lack of density of Lipschitz mappings in Sobolev spaces with Heisenberg target

Noel DeJarnette, Piotr Hajlasz, Anton Lukyanenko, Jeremy Tyson

摘要:

研究了Sobolev空间$W^{1,p}(M,\mathbf{H}^n)$中Lipschitz映射何时稠密的问题。这里$M$表示有边界或无边界的紧致黎曼流形,而$\mathbf{H}^n$表示具有次黎曼度量的$n$阶海森堡群.我们证明了Lipschitz映射在$W^{1,p}(M,\mathbf{H}^n)$中是稠密的,对于所有的$1\le p<\infty$,如果$\dim M \le n$,但Lipschitz映射在$W^{1,p}(M,\mathbf{H}^n)$中不是稠密的,如果$\dim M \ge n+1$和$n\le p<n+1$。证明依赖于球体$S^n$到$\mathbf{H}^n$的平滑水平嵌入的构造。我们提供了两个这样的结构,一个产生于复杂的双曲几何和其他辛几何。非密度断言可以解释为$\mathbf{H}^n$的$n$第Lipschitz同伦群的非平凡性。本文研究了次黎曼空间的Lipschitz同伦群。

英文摘要:

We study the question: when are Lipschitz mappings dense in the Sobolev space $W^{1,p}(M,\mathbf{H}^n)$? Here $M$ denotes a compact Riemannian manifold with or without boundary, while $\mathbf{H}^n$ denotes the $n$th Heisenberg group equipped with a sub-Riemannian metric. We show that Lipschitz maps are dense in $W^{1,p}(M,\mathbf{H}^n)$ for all $1\le p<\infty$ if $\dim M \le n$, but that Lipschitz maps are not dense in $W^{1,p}(M,\mathbf{H}^n)$ if $\dim M \ge n+1$ and $n\le p<n+1$. The proofs rely on the construction of smooth horizontal embeddings of the sphere $S^n$ into $\mathbf{H}^n$. We provide two such constructions, one arising from complex hyperbolic geometry and the other arising from symplectic geometry. The nondensity assertion can be interpreted as nontriviality of the $n$th Lipschitz homotopy group of $\mathbf{H}^n$. We initiate a study of Lipschitz homotopy groups for sub-Riemannian spaces.

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