Almost Flat Manifold

In mathematics, a smooth compact manifold M is called almost flat if for any \epsilon>0 there is a Riemannian metric g_\epsilon on M such that \mbox{diam}(M,g_\epsilon)\le 1 and g_\epsilon is \epsilon-flat, i.e. for sectional curvature of K_{g_\epsilon} we have |K_{g_\epsilon}|<\epsilon. In fact, given n, there is a positive number \epsilon_n>0 such that if a n-dimensional manifold admits an \epsilon_n-flat metric with diameter \le 1 then it is almost flat. 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 nill manifold, i.e. a total space of a oriented circle bundle over a oriented circle bundle over ... over a circle.

 

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