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arXiv 0910.3872math.DG
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关于非紧调和流形的新结果

New results on noncompact harmonic manifolds

Gerhard Knieper

摘要:

Lichnerowicz猜想断言所有的调和流形要么是平坦的要么是秩为1的局部对称空间。这个猜想已被Z. Szabó \cite{Sz}:具有紧万有覆盖的调和流形. E. Damek和F.利玛窦\cite{DR}提供的例子表明,在非紧的情况下,猜想是错误的。然而,这样的流形不允许紧商。本文利用秩的概念研究了单连通非平坦非紧调和流形$X$上的渐近几何和测地流。在本文的第一部分中,我们证明了以下断言是等价的。体积的增长是纯指数的,$X$的秩是1,测地流是Anosov关于Sasaki度量,$X$是Gromov双曲。本文第二部分证明了若$X$是一个无焦点的非平坦调和流形,则测地流是Anosov流。在证明的过程中,我们得到了任意黎曼流形上的某些部分双曲流是Anosov流,这是调和流形以外的有趣的。结合\cite{BCG},\cite{BFL}和\cite{FL}的刚性定理,我们对所有无焦点或含Gromov双曲基本群的紧致调和流形证实了Lichnerowicz猜想.

英文摘要:

The Lichnerowicz conjecture asserts that all harmonic manifolds are either flat or locally symmetric spaces of rank~1. This conjecture has been proved by Z. Szabó \cite{Sz} for harmonic manifolds with compact universal cover. E. Damek and F. Ricci \cite{DR} provided examples showing that in the noncompact case the conjecture is wrong. However, such manifolds do not admit a compact quotient. In this paper we study, using a notion of rank, the asymptotic geometry and the geodesic flow on simply connected nonflat and noncompact harmonic manifolds denoted by $X$. In the first part of the paper we show that the following assertions are equivalent. The volume growth is purely exponential, the rank of $X$ is one, the geodesic flow is Anosov with respect to the Sasaki metric, $X$ is Gromov hyperbolic. In the second part of the paper we show that the geodesic flow is Anosov if $X$ is a nonflat harmonic manifold with no focal points. In the course of the proof we obtain that certain partially hyperbolic flows on arbitrary Riemannian manifolds without focal points are Anosov, which is of interest beyond harmonic manifolds. Combining the results of this paper with the rigidity theorem's of \cite{BCG}, \cite{BFL} and \cite{FL}, we confirm the Lichnerowicz conjecture for all compact harmonic manifolds without focal points or with Gromov hyperbolic fundamental groups.

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