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Almost flat manifold : ウィキペディア英語版
Almost flat manifold
In mathematics, a smooth compact manifold ''M'' is called almost flat if for any \varepsilon>0 there is a Riemannian metric g_\varepsilon on ''M'' such that \mbox(M,g_\varepsilon)\le 1 and
g_\varepsilon is \varepsilon-flat, i.e. for the sectional curvature of K_ we have |K_| < \varepsilon.
In fact, given ''n'', there is a positive number \varepsilon_n>0 such that if a ''n''-dimensional manifold admits an \varepsilon_n-flat metric with diameter \le 1 then it is almost flat. On the other hand one can fix the bound of sectional curvature and get the diameter going to zero, so the almost flat manifold is a special case of a collapsing manifold, which is collapsing along all directions.
According to the Gromov—Ruh theorem, ''M'' is almost flat if and only if it is infranil. In particular, it is a finite factor of a nilmanifold, which is the total space of a principal torus bundle over a principal torus bundle over a torus.
==References==

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抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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