Are group schemes in Char 0 reduced? (YES) - MathOverflow

Are group schemes in Char 0 reduced? (YES)

2010-04-26T02:27:34Z

A Theorem of Cartier (e.g. Mumford, Lecture 25) states that every separated, finite type group scheme $G/k$ over a field $k$ of characteristic $0$ is reduced. Does this result remain valid if we drop the assumption that $G/k$ is separated and of finite type?

Frans Oort (MR0206005) observed that one can use limit formalism to argue that every affine group scheme over $k$ is reduced.

Edit: BCnrd pointed out that group schemes over a field are automatically separated. Furthermore, the proof of Cartier's Theorem in Mumford's book remains valid for a locally Noetherian $k$-group scheme.

Answer by Harry Gindi for Are group schemes in Char 0 reduced? (YES)

2010-04-26T03:04:16Z

BCnrd posted:

Every group scheme over a field is separated: rational points are closed immersions, and the diagonal is the base change of the identity section. Also, a connected group locally of finite type over field k is of finite type (use geometric connectedness and pass to the algebraic closure of k), whence smoothness follows for characteristic 0 in the locally of finite type case. (The proof of Cartier's theorem works in the locally of finite type case over a field of characteristic 0, so this reasoning is silly.) Any noetherian group scheme over field of characteristic 0 is formally smooth: the completion at 1 is a formal group of finite dimension, and Cartier's proof works in formal case (use formal Lie theory without a smoothness hypothesis!), or use Theorem 3.3ff Exp. VII of SGA3. Then translate and extend base field. QED

Answer by LRG for Are group schemes in Char 0 reduced? (YES)

2010-04-26T07:59:12Z

The answer is yes - every group scheme over a field of characterstic zero is reduced: see Schémas en groupes quasi-compacts sur un corps et groupes henséliens (especially Thm. 2.4 in part II and Thm. 1.1 and Cor. 3.9 in part V of the 1st part), and for a summary of the relevant results see 4.2 (in particular 4.2.8) of Approximation des schémas en groupes, quasi compacts sur un corps.