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Closed immersion
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Closed immersion : ウィキペディア英語版
Closed immersion

:''For the same-name concept in differential geometry, see immersion (mathematics).''
In algebraic geometry, a closed immersion of schemes is a morphism of schemes f: Z \to X that identifies ''Z'' as a closed subset of ''X'' such that regular functions on ''Z'' can be extended locally to ''X''.〔Mumford, ''The Red Book of Varieties and Schemes'', Section II.5〕 The latter condition can be formalized by saying that f^\#:\mathcal_X\rightarrow f_\ast\mathcal_Z is surjective.
An example is the inclusion map \operatorname(R/I) \to \operatorname(R) induced by the canonical map R \to R/I.
==Other characterizations==

The following are equivalent:
#f: Z \to X is a closed immersion.
#For every open affine U = \operatorname(R) \subset X, there exists an ideal I \subset R such that f^(U) = \operatorname(R/I) as schemes over ''U''.
#There exists an open affine covering X = \bigcup U_j, U_j = \operatorname R_j and for each ''j'' there exists an ideal I_j \subset R_j such that f^(U_j) = \operatorname (R_j / I_j) as schemes over U_j.
#There is a quasi-coherent sheaf of ideals \mathcal on ''X'' such that f_\ast\mathcal_Z\cong \mathcal_X/\mathcal and ''f'' is an isomorphism of ''Z'' onto the global Spec of \mathcal_X/\mathcal over ''X''.

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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