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bio website perso.univ-rennes1.fr/…
location Rennes
age 38
visits member for 4 years, 7 months
seen 19 hours ago

2d
comment How do you calculate the group scheme of E[p] for a an elliptic curve E in characteristic p?
In your first sentence, the p-torsion kernel $E[p]$ of a supersingular elliptic curve is not $\alpha_p$. Indeed, the former has order $p^2$.
Dec
9
comment Quotient of a (non-linear) algebraic group by a closed subgroup
Also Perrin, Approximation des schémas en groupes quasi-compacts sur un corps, Bull. SMF 1976, shows the following: if G is quasi-compact and H is either a normal subgroup, or defined by a finitely generated ideal, then the fpqc sheaf G/H is representable by a scheme.
Dec
3
answered Regular rings and formally smooth algebras
Nov
16
revised Is this formally étale morphism of schemes an isomorphism?
deleted 193 characters in body
Nov
15
revised Is this formally étale morphism of schemes an isomorphism?
added 5 characters in body
Nov
15
answered Is this formally étale morphism of schemes an isomorphism?
Nov
8
comment Is this formally étale morphism of schemes an isomorphism?
Two questions: 1) I'd be happy to have details on the proof that $\mathfrak{m}'=\mathfrak{m}B$. 2) For formal glueing in the very last step, the quoted lemma from the Stacks Project requires to know that $B$ is a finite $A$-module, how do you deal with this issue?
Nov
4
comment Fantastic properties of Z/2Z
That is terrific! Do you know a reference where I can read more on this?
Nov
3
accepted Is this formally étale morphism of schemes an isomorphism?
Nov
3
comment Is this formally étale morphism of schemes an isomorphism?
OK, I get it. Thank you for this nice contribution. The particular situation that I have in mind has additional features (like quasicompactness) that your example doesn't, but it helped me anyway to understand things better. Thanks again! (And if you happen to have ideas in the quasicompact case...)
Nov
3
revised Is this formally étale morphism of schemes an isomorphism?
Added assumptions to the original question to fit with the actual situation I'm interested in.
Nov
3
comment Is this formally étale morphism of schemes an isomorphism?
Why it is true that $f$ is an iso on local rings at points of $Y$?
Nov
3
comment Is this formally étale morphism of schemes an isomorphism?
... and $S$ is noetherian.
Nov
3
comment Is this formally étale morphism of schemes an isomorphism?
In view of the example given by user52824 below, I wish to emphasize my comment 1: I am in fact interested in the case where $f$ is quasicompact.
Nov
2
asked Is this formally étale morphism of schemes an isomorphism?
Nov
2
awarded  Notable Question
Nov
2
awarded  Good Question
Oct
17
answered Wonderful applications of the Vandermonde determinant
Oct
13
comment The quotient stack $[\mathbb{A}^n / \mathrm{GL}_n]$
@André, Ben: you're perfectly right of course.
Oct
12
comment The quotient stack $[\mathbb{A}^n / \mathrm{GL}_n]$
This stack classifies vector bundles of rank $n$ together with $n$ global sections. You see it just like you see that $A^1/G_m$ classifies line bundles with a global section.