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arXiv 1104.0856math.AGmath.NT
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伪阿贝尔簇

Pseudo-abelian varieties

Burt Totaro

摘要:

Chevalley定理指出,完满域上的每个光滑连通代数群都是光滑连通仿射群对阿贝尔簇的扩张。当基本场不完美时,它就失败了。定义域k上的伪阿贝尔簇为光滑连通k-群,其中每个光滑连通仿射法线k-子群是平凡的.这给代数群的分类提供了一个新的观点:域上的每个光滑连通群都是伪阿贝尔簇通过光滑连通仿射群的唯一扩张.我们研究了伪阿贝尔簇的许多结构。这些群与特徴为p的幂幺群和Tits和Conrad-Gabber-Prasad研究的伪约化群密切相关。阿贝尔簇的许多性质如Mordell-Weil定理可推广到伪阿贝尔簇。最后,我们在Milnor猜想的基础上,利用生成元和关系猜想了任意域上Ext^2(G_a,G_m)的一个刻画。

英文摘要:

Chevalley's theorem states that every smooth connected algebraic group over a perfect field is an extension of an abelian variety by a smooth connected affine group. That fails when the base field is not perfect. We define a pseudo-abelian variety over an arbitrary field k to be a smooth connected k-group in which every smooth connected affine normal k-subgroup is trivial. This gives a new point of view on the classification of algebraic groups: every smooth connected group over a field is an extension of a pseudo-abelian variety by a smooth connected affine group, in a unique way. We work out much of the structure of pseudo-abelian varieties. These groups are closely related to unipotent groups in characteristic p and to pseudo-reductive groups as studied by Tits and Conrad-Gabber-Prasad. Many properties of abelian varieties such as the Mordell-Weil theorem extend to pseudo-abelian varieties. Finally, we conjecture a description of Ext^2(G_a,G_m) over any field by generators and relations, in the spirit of the Milnor conjecture.

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