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Algebraic varieties with group operations given by morphisms, or group objects in the category of algebraic varieties, the category of algebraic schemes, or closely related categories.

4 votes

Does every morphism BG-->BH come from a homomorphism G-->H?

As a complement to the answers above : it is kind of well-known (at least I thought it was) that the natural morphism $$\operatorname{\mathbf{Hom}}_{gr} (G,H) \to \operatorname{\mathbf{Hom}}(BG,BH)$ …
Niels's user avatar
  • 4,008
2 votes

Spin group as an automorphism group

See Groupes Classiques Baptiste Calmès, Jean Fasel http://arxiv.org/abs/1401.1992 Propositions 4.5.1.15 and 4.5.1.16.
Niels's user avatar
  • 4,008
6 votes

When quotient stacks (for nonsmooth group) are algebraic and related questions

About 1. : no, smoothness isn't essential. "Flat is enough" : De Jong's slogan to express this result due to M.Artin. https://www.math.columbia.edu/~dejong/wordpress/?p=1584 I quote : "Given a flat, f …
Niels's user avatar
  • 4,008
5 votes

Are principal bundles isotrivial?

This answer is coming late, but since I have also been struggling to find a reference, I hope this can be helpful to other people. The answer is yes. Precisely, you can found a proof in Raynaud, Mi …
Niels's user avatar
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