bio  website  perso.univrennes1.fr/… 

location  Rennes  
age  38  
visits  member for  4 years, 7 months 
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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 ptorsion 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 (nonlinear) algebraic group by a closed subgroup
Also Perrin, Approximation des schémas en groupes quasicompacts sur un corps, Bull. SMF 1976, shows the following: if G is quasicompact 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. 