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for questions about deformation theory, including deformations of manifolds, schemes, Galois representations, and von Neumann algebras.

19 votes
4 answers
2k views

Example of a smooth morphism where you can't lift a map from a nilpotent thickening?

Definition. A locally finitely presented morphism of schemes $f\colon X\to Y$ is smooth (resp. unramified, resp. étale) if for any affine scheme $T$, any closed subscheme $T_0$ defined by a square …
Anton Geraschenko's user avatar
25 votes
4 answers
2k views

algebraic group G vs. algebraic stack BG

I've gathered that it's "common knowledge" (at least among people who think about such things) that studying a (smooth) algebraic group G, as an algebraic group, is in some sense the same as studying …
Anton Geraschenko's user avatar
30 votes
5 answers
4k views

Deformation theory of representations of an algebraic group

For an algebraic group G and a representation V, I think it's a standard result (but I don't have a reference) that the obstruction to deforming V as a representation of G is an element of H2(G,V⊗V* …
Anton Geraschenko's user avatar