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local flatness : ウィキペディア英語版
local flatness
In topology, a branch of mathematics, local flatness is a property of a submanifold in a topological manifold of larger dimension. In the category of topological manifolds, locally flat submanifolds play a role similar to that of embedded submanifolds in the category of smooth manifolds.
Suppose a ''d'' dimensional manifold ''N'' is embedded into an ''n'' dimensional manifold ''M'' (where ''d'' < ''n''). If x \in N, we say ''N'' is locally flat at ''x'' if there is a neighborhood U \subset M of ''x'' such that the topological pair (U, U\cap N) is homeomorphic to the pair (\mathbb^n,\mathbb^d), with a standard inclusion of \mathbb^d as a subspace of \mathbb^n. That is, there exists a homeomorphism U\to R^n such that the image of U\cap N coincides with \mathbb^d.
The above definition assumes that, if ''M'' has a boundary, ''x'' is not a boundary point of ''M''. If ''x'' is a point on the boundary of ''M'' then the definition is modified as follows. We say that ''N'' is locally flat at a boundary point ''x'' of ''M'' if there is a neighborhood U\subset M of ''x'' such that the topological pair (U, U\cap N) is homeomorphic to the pair (\mathbb^n_+,\mathbb^d), where \mathbb^n_+ is a standard half-space and \mathbb^d is included as a standard subspace of its boundary. In more detail, we can set
\mathbb^n_+ = \ and \mathbb^d = \=\cdots=y_n=0\}.
We call ''N'' locally flat in ''M'' if ''N'' is locally flat at every point. Similarly, a map \chi\colon N\to M is called locally flat, even if it is not an embedding, if every ''x'' in ''N'' has a neighborhood ''U'' whose image \chi(U) is locally flat in ''M''.
Local flatness of an embedding implies strong properties not shared by all embeddings. Brown (1962) proved that if ''d'' = ''n'' − 1, then ''N'' is collared; that is, it has a neighborhood which is homeomorphic to ''N'' × () with ''N'' itself corresponding to ''N'' × 1/2 (if ''N'' is in the interior of ''M'') or ''N'' × 0 (if ''N'' is in the boundary of ''M'').
==See also==

*Neat submanifold

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「local flatness」の詳細全文を読む



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