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Immersion (mathematics) : ウィキペディア英語版
Immersion (mathematics)

:''For a closed immersion in algebraic geometry, see closed immersion.''
In mathematics, an immersion is a differentiable function between differentiable manifolds whose derivative is everywhere injective.〔This definition is given by , , , , , , , .〕 Explicitly, ''f'' : ''M'' → ''N'' is an immersion if
:D_pf : T_p M \to T_N\,
is an injective function at every point ''p'' of ''M'' (where ''TpX'' denotes the tangent space of a manifold ''X'' at a point ''p'' in ''X''). Equivalently, ''f'' is an immersion if its derivative has constant rank equal to the dimension of ''M'':〔This definition is given by , .〕
:\operatorname\,D_p f = \dim M.
The function ''f'' itself need not be injective, only its derivative.
A related concept is that of an embedding. A smooth embedding is an injective immersion ''f'' : ''M'' → ''N'' which is also a topological embedding, so that ''M'' is diffeomorphic to its image in ''N''. An immersion is precisely a local embedding – i.e. for any point ''x'' ∈ ''M'' there is a neighbourhood, ''U'' ⊂ ''M'', of ''x'' such that ''f'' : ''U'' → ''N'' is an embedding, and conversely a local embedding is an immersion.〔This kind of definition, based on local diffeomorphisms, is given by , .〕 For infinite dimensional manifolds, this is sometimes taken to be the definition of an immersion.〔This kind of infinite-dimensional definition is given by .〕
If ''M'' is compact, an injective immersion is an embedding, but if ''M'' is not compact then injective immersions need not be embeddings; compare to continuous bijections versus homeomorphisms.
==Regular homotopy==
A regular homotopy between two immersions ''f'' and ''g'' from a manifold ''M'' to a manifold ''N'' is defined to be a differentiable function ''H'' : ''M'' × () → ''N'' such that for all ''t'' in (1 ) the function ''Ht'' : ''M'' → ''N'' defined by ''Ht''(''x'') = ''H''(''x'', ''t'') for all ''x'' ∈ ''M'' is an immersion, with ''H''0 = ''f'', ''H''1 = ''g''. A regular homotopy is thus a homotopy through immersions.

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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