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Sphere theorem (3-manifolds) : ウィキペディア英語版
Sphere theorem (3-manifolds)

In mathematics, in the topology of 3-manifolds, the sphere theorem of gives conditions for elements of the second homotopy group of a 3-manifold to be represented by embedded spheres.
One example is the following:
Let M be an orientable 3-manifold such that \pi_2(M) is not the trivial group. Then there exists a non-zero element of
\pi_2(M) having a representative that is an embedding S^2\to M.
The proof of this version can be based on transversality methods, see Batude below.
Another more general version (also called the projective plane theorem due to Epstein) is:
Let M be any 3-manifold and N a \pi_1(M)-invariant subgroup of \pi_2(M). If f\colon S^2\to M is a general position map such that ()\notin N and U is any neighborhood of the singular set \Sigma(f), then there is a map g\colon S^2\to M satisfying
#()\notin N,
#g(S^2)\subset f(S^2)\cup U,
#g\colon S^2\to g(S^2) is a covering map, and
#g(S^2) is a 2-sided submanifold (2-sphere or projective plane) of M.

quoted in Hempel (p. 54)
==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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