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In algebraic geometry, given a smooth projective curve ''X'' over a finite field and a smooth affine group scheme ''G'' over it, the moduli stack of principal bundles over ''X'', denoted by , is an algebraic stack given by:〔http://www.math.harvard.edu/~lurie/283notes/Lecture2-FunctionFields.pdf〕 for any -algebra ''R'', : the category of principal ''G''-bundles over the relative curve . In particular, the category of -points of , that is, , is the category of ''G''-bundles over ''X''. Similarly, can also be defined when the curve ''X'' is over the field of complex numbers. Roughly, in the complex case, one can define as the quotient stack of the space of holomorphic connections on ''X'' by the gauge group. Replacing the quotient stack (which is not a topological space) by a homotopy quotient (which is a topological space) gives the homotopy type of . In the finite field case, it is not common to define the homotopy type of . But one can still define a (smooth) cohomology and homology of . == Basic properties == It is known that is a smooth stack of dimension where is the genus of ''X''. It is not of finite type but of locally of finite type; one thus usually uses a stratification by open substacks of finite type (cf. the Harder–Narasimhan stratification.) If ''G'' is a split reductive group, then the set of connected components is in a natural bijection with the fundamental group . 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「moduli stack of principal bundles」の詳細全文を読む スポンサード リンク
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