Friday, 2 November 2007

ag.algebraic geometry - Is every homogeneous G-variety of the form G/H?

It depends on what you mean by "closed subgroup". If you mean a Zariski closed subset which forms a subgroup then the answer is no. If you mean a closed subgroup scheme, then the answer is yes. An example where you need to use the second definition is the Frobenius map FcolonGtoG(p). If we let G act on G(p) through F then the action is transitive and indeed G(p) is isomorphic to G/KerF. However, unless G is zero-dimensional KerF is a non-trivial finite group scheme whose k-points consist of just the identity.



Note however that G/H is always quasi-projective even when H is a subgroup scheme so all homogeneous G-spaces are quasi-projective.

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