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Groupoid scheme : ウィキペディア英語版
Groupoid scheme
In algebraic geometry, a groupoid scheme is a pair of schemes R, U together with five morphisms s, t: R \to U, e: U \to R, m: R \times_ R \to R, i: R \to R satisfying s \circ e, t \circ e are the identity morphisms, s \circ m = s \circ p_1, t \circ m = t \circ p_2 and other obvious conditions that generalize the axioms of group action; e.g., associativity. In practice, it is usually written as R \rightrightarrows U (cf. coequalizer.)
Example: Suppose an algebraic group ''G'' acts from the right on a scheme ''U''. Then take R = U \times G, ''s'' the projection, ''t'' the given action.
The main use of the notion is that it provides an atlas for a stack. More specifically, let (\rightrightarrows U ) be the category of (R \rightrightarrows U)-torsors. Then it is a category fibered in groupoids; in fact, a Deligne–Mumford stack. Conversely, any DM stack is of this form.
== Notes ==


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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