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Regular embedding : ウィキペディア英語版
Regular embedding
In algebraic geometry, a closed immersion i: X \hookrightarrow Y of schemes is a regular embedding of codimension ''r'' if each point ''x'' in ''X'' has an open affine neighborhood ''U'' in ''Y'' such that the ideal of X \cap U is generated by a regular sequence of length ''r''.
For example, if ''X'' and ''Y'' are smooth over a scheme ''S'' and if ''i'' is an ''S''-morphism, then ''i'' is a regular embedding. In particular, every section of a smooth morphism is a regular embedding. If \operatornameB is regularly embedded into a regular scheme, then ''B'' is a complete intersection ring.
The notion is used, for instance, in an essential way in Fulton's approach to intersection theory. The important fact is that when ''i'' is a regular embedding, if ''I'' is the ideal sheaf of ''X'' in ''Y'', then the normal sheaf, the dual of I/I^2, is locally free (thus a vector bundle) and the natural map \operatorname(I/I^2) \to \oplus_0^\infty I^n/I^ is an isomorphism: the normal cone \operatorname(\oplus_0^\infty I^n/I^) coincides with the normal bundle.
A flat morphism of finite type f:X \to Y is called a (local) complete intersection morphism if each point ''x'' in ''X'' has an open affine neighborhood ''U'' so that ''f'' |''U'' factors as U \overset\to V \overset\to Y where ''j'' is a regular embedding and ''g'' is smooth. For example, if ''f'' is a morphism between smooth varieties, then ''f'' factors as X \to X \times Y \to Y where the first map is the graph morphism and so is a complete intersection morphism.
== References ==

*, section B.7
*E. Sernesi: ''(Deformations of algebraic schemes )''


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