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In algebraic geometry, given a category ''C'', a categorical quotient of an object ''X'' with action of a group ''G'' is a morphism that :(i) is invariant; i.e., where is the given group action and ''p''2 is the projection. :(ii) satisfies the universal property: any morphism satisfying (i) uniquely factors through . One of the main motivations for the development of geometric invariant theory was the construction of a categorical quotient for varieties or schemes. Note need not be surjective. Also, if it exists, a categorical quotient is unique up to a canonical isomorphism. In practice, one takes ''C'' to be the category of varieties or the category of schemes over a fixed scheme. A categorical quotient is a universal categorical quotient if it is stable under base change: for any , is a categorical quotient. A basic result is that geometric quotients (e.g., ) and GIT quotients (e.g., ) are categorical quotients. == References == * Mumford, David; Fogarty, J.; Kirwan, F. ''Geometric invariant theory''. Third edition. Ergebnisse der Mathematik und ihrer Grenzgebiete (2) (Results in Mathematics and Related Areas (2)), 34. Springer-Verlag, Berlin, 1994. xiv+292 pp. ISBN 3-540-56963-4 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「Categorical quotient」の詳細全文を読む スポンサード リンク
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