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Affine Grassmannian : ウィキペディア英語版 | Affine Grassmannian
In mathematics, the term affine Grassmannian has two distinct meanings; the concept treated in this article is the affine Grassmannian of an algebraic group ''G'' over a field ''k''. It is an ind-scheme—a limit of finite-dimensional schemes—which can be thought of as a flag variety for the loop group ''G(k((t)))'' and which describes the representation theory of the Langlands dual group ''L''''G'' through what is known as the geometric Satake correspondence. == Definition of Gr via functor of points ==
Let ''k'' be a field, and denote by and the category of commutative ''k''-algebras and the category of sets respectively. Through the Yoneda lemma, a scheme ''X'' over a field ''k'' is determined by its functor of points, which is the functor which takes ''A'' to the set ''X(A)'' of ''A''-points of ''X''. We then say that this functor is representable by the scheme ''X''. The affine Grassmannian is a functor from ''k''-algebras to sets which is not itself representable, but which has a filtration by representable functors. As such, although it is not a scheme, it may be thought of as a union of schemes, and this is enough to profitably apply geometric methods to study it. Let ''G'' be an algebraic group over ''k''. The affine Grassmannian Gr''G'' is the functor that associates to a ''k''-algebra ''A'' the set of isomorphism classes of pairs (''E'', ''φ''), where ''E'' is a principal homogeneous space for ''G'' over Spec ''A'' and ''φ'' is an isomorphism, defined over Spec ''A''((''t'')), of ''E'' with the trivial ''G''-bundle ''G'' × Spec ''A''((''t'')). By the Beauville–Laszlo theorem, it is also possible to specify this data by fixing an algebraic curve ''X'' over ''k'', a ''k''-point ''x'' on ''X'', and taking ''E'' to be a ''G''-bundle on ''X''''A'' and ''φ'' a trivialization on (''X'' − ''x'')''A''. When ''G'' is a reductive group, Gr''G'' is in fact ind-projective, i.e., an inductive limit of projective schemes.
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「Affine Grassmannian」の詳細全文を読む
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