Questions tagged [group-schemes]
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72
questions with no upvoted or accepted answers
16
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answers
412
views
Complete resource of Ngô's course notes on Algebraic Groups and Group Schemes
I'm looking for Ngô's M2 course notes on "Groupes algébriques et schémas en groupes".
The Wayback Machine has an incomplete capture here. However, it apparently lacks chapter 1, 3, and 5.
I ...
12
votes
0
answers
344
views
Is the quotient of two linear group schemes linear?
Let $S$ be an affine scheme.
Call a group scheme $G\to S$ linear if there exists an $S$-group morphism $G\to \mathrm{GL}_{n,S}$ with trivial kernel.
Assuming this, suppose $H\to S$ is a central closed ...
9
votes
0
answers
466
views
Classification of finite flat group schemes over integers?
One can classify (commutative) finite flat group schemes (with order of $p$-powers) over $\mathbb Z_p$ using semi-linear algebraic datas such as Breuil-Kisin modules. And we can fix the special fiber ...
8
votes
0
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253
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Global functions on a product of schemes over artinian ring
For a morphism of schemes $f:X\to S$ with $S=\text{Spec}(R)$ affine, let's write $A(X)=H^0(X,\mathcal{O}_X)$. I'm interested in the morphism of $R$-algebras
$$
c:A(X)\otimes_R A(Y)\to A(X\times_SY)
$$
...
7
votes
0
answers
153
views
Are maximal tori conjugate Zariski-locally?
Let $S$ be a scheme and let $G\to S$ be a reductive group scheme. Then $G$ admits a maximal torus etale-locally, and any two maximal tori are conjugate etale-locally, by Theorem 3.2.6 and Corollary 3....
7
votes
0
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359
views
When is the character group scheme of a group scheme representable? (Affine Case)
While reading Tate's article on Finite Flat Group Schemes in "Modular Forms and Fermat's Last Theorem" I was lead to this question. Let $S$ be a scheme, $G$ a group scheme over $S$, and $T$ an $S$-...
7
votes
0
answers
893
views
Deformation of ordinary p-divisible groups via Grothendieck-Messing
I am hoping that someone can point out the error in the "proof" of the following "theorem":
Theorem: Let $k$ be a perfect field of characteristic $p>2$ and let $G$ be an ordinary $p$-divisible ...
6
votes
0
answers
236
views
Semistable model of product of modular curves
Does the product $Y_1(Np) \times Y_1(Np)$ admit a semistable model over $\mathbf{Z}_p[\zeta_p]$ with a natural moduli-space interpretation?
Less telegraphically: let $p$ be a prime, and $N \ge 4$ ...
6
votes
0
answers
570
views
Ext group for commutative finite group schemes
EDIT: all group schemes are commutative. Thanks @R. van Dobben de Bruyn!
Could anyone provide a reference request about extensions of finite group schemes / Ext groups.
As far as I know the category ...
6
votes
0
answers
231
views
Can we classify reductive group schemes over curves
Let $C$ be a smooth quasi-projective connected curve over the complex numbers.
Can one classify all reductive group schemes over $C$?
Certainly, you have the trivial ones (coming from pulling-back ...
5
votes
0
answers
143
views
Faltings' Cartier duality for A-modules in terms of Hopf algebras
$\newcommand\dual{^{\text{dual}}}\newcommand\GrpSch{\mathrm{GrpSch}}\DeclareMathOperator\Hom{Hom}\DeclareMathOperator\Vect{Vect}$If $G$ is a finite group scheme over a field $k$, we can define its ...
5
votes
0
answers
141
views
Finite locally free group scheme killed by its order?
When I saw this paper of René Schoof, there are two questions on the first page and what confuses me is that how to reduce the first question to the second. This is expalined in the first paragraph on ...
5
votes
0
answers
306
views
motivations of classifying $p$-divisible groups
Let $k$ be a perfect field of characteristic $p>0$ and $W:=W(k)$ is the witt ring. Let $K$ be a totally ramified extension of $K_0:=W(\frac{1}{p})$ and $\Lambda:=W[[u]]$ is the formal series ring ...
5
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237
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Semiabelian actions appearing in the toroidal campactification of a degenearting abelian varieties
Given a totally degenerated abelian variety $A_K$ (to make it easier) over a complete discrete valuation field $K$ with $R$, $\pi$ and $k$ the corresponding discrete valuation ring, uniformiser and ...
4
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206
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Do rational maps to abelian varieties extend across rational singularities?
Let $X$ be a normal proper variety with only rational singularities and $A$ an abelian variety. Does a rational map $X \supset U \to A$ extend to a morphism $X \to A$? If not, what is a ...
4
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0
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225
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Lifting the connected-etale sequence of the $p$-torsion of an elliptic curve
Suppose that $R$ is a complete DVR with characteristic 0 fraction field $K$, maximal ideal generated by $p$ and characteristic $p>0$ residue field $k$ which is algebraically closed. Suppose that $\...
4
votes
0
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293
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Reference request for Kummer-Artin-Schreier-Witt theory
I cannot find the following 4 papers by Sekiguchi–Suwa in their works on Kummer–Artin–Schreier–Witt theory:
On the unified Kummer–Artin–Schreier–Witt theory, Prépublications du laboratoire de ...
4
votes
0
answers
280
views
An application of Grothendieck's version of Hensel's Lemma
Assume $R$ is a Henselian local ring with $k=R/m_R$ ($m_R$ is the unique maximal ideal of $R$) and $G$ a finite flat group scheme over $R$. We denote by $G_k= G \otimes_R k$ the closed fiber.
There ...
4
votes
0
answers
325
views
A question about Weil restriction
Let $\pi:\tilde{C}\rightarrow C$ be a ramified cover between two smooth curves. And consider a group scheme $\mathcal G$ over $\tilde{C}$, I have found two definitions for Weil restriction:
$Res_{\...
4
votes
0
answers
217
views
An extension of group schemes admitting Neron models
Let $R$ be a discrete valuation ring, $K$ its field of fractions, and
$$ 0 \rightarrow G_K' \rightarrow G_K \rightarrow G_K'' \rightarrow 0$$
a short exact sequence of smooth $K$-group schemes of ...
4
votes
0
answers
159
views
Is this $S$-birational map an open immersion on its domain of definition?
My question is about a claim on the bottom of p. 121 of the book "Neron models" by Bosch, Lutkebohmert, and Raynaud, so I will freely use the general terminology recalled in this book, but will ...
4
votes
0
answers
318
views
Tannaka categories and reductive groups
The group associated to a Tannaka category $T$ over a field is pro-reductive if and only if $T$ is semi-simple.
Pro-reductive groups make sense over any scheme.
Is there an extension of the theory ...
4
votes
0
answers
604
views
A 'standard patching argument' in Mazur's Eisenstein Ideal paper
On pp 46 of his Eisenstein Ideal paper, Mazur states Theorem I.4 and in the discussion that follows, he mentions 'a standard patching argument' that completes the proof. I was wondering whether this ...
3
votes
0
answers
250
views
Does the orbit in geometric invariant theory have natural scheme structure
Let $X$ be a scheme locally of finite type over a sufficiently "nice" base scheme $S$ (nice in sense of reasonable "finiteness conditions", for sake of simplicity let's start as ...
3
votes
0
answers
96
views
Noncommutative group schemes corresponding to quantum groups
I'm not an expert on quantum groups by any stretch, so forgive me if this question seems overly naive. That said, I was wondering if there is a way (or if there has been any attempt in the literature) ...
3
votes
0
answers
72
views
Is the product of unipotent radicals of opposite Borels a closed immersion?
Let $G$ be a reductive group over a scheme $S$ and let $B \subset G$ and $B' \subset G$ be opposite Borel subgroups with their unipotent radicals $U \subset B \subset G$ and $U' \subset B' \subset G$. ...
3
votes
0
answers
171
views
A question regarding Corollary 4.12 in Mumford's "Analytic construction of deg. Ab. Var."
Let $S=Spec A$ be the spectrum of an integrally closed, excellent and Noetherian ring. In his paper, David Mumford constructs an $S$-group scheme $G$. He shows that the torsion of $G_s$ is $p$-...
3
votes
0
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147
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Finite commutative group schemes whose exponent coincides with its rank
In group theory, a finite commutative group $G$ contains an element whose order is the exponent of $G$. Thus, If the exponent of $G$ is the same as the order of $G$, it must be that $G$ is cyclic. ...
3
votes
0
answers
152
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Can one embed a group scheme into a locally constant one such that the quotient exists
Let $S$ be a good enough base scheme (say of finite type over an algebraic closed field) and $G\to S$ be a flat group scheme. I want to ask: can we always find a closed embedding $G\to H$ into another ...
3
votes
0
answers
208
views
Derived subgroup of the connected component of an algebraic group
Let $G$ be an affine group variety (smooth) over a field $k$, let $G^0$ be the connected component of $G$.
Is it true that the derived subgroup $[G^0,G^0]$ (Or $[G,G^0]$) is the connected component ...
3
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0
answers
179
views
component group of Neron models
Let $A$ be an abelian variety over a discretely valued field $K$ and $\mathcal A$ its Neron model over $R$ (the ring of integers of $K$) and $\mathcal A^0$ the identity component of $\mathcal A$.
...
3
votes
0
answers
165
views
Simply-connected covers of group schemes
Let $\mathbf G$ be a semisimple group scheme over a base $S$. I believe that one can always find a fiberwise simply-connected group scheme $\mathbf G^{sc}$ and a central isogeny $\mathbf G^{sc}\to\...
3
votes
0
answers
296
views
Quotient of a sheaf by group action and representabillity
Let $X$ be a scheme and $S$ be a sheaf of sets over the fppf topology of $X$. Let $G$ be a group scheme over $X$ and there is an action of $G$ on $S$. Now, I want to look at the quotient $G \setminus ...
3
votes
0
answers
435
views
"Frobenius Descent"
The following proposition is there in Pink's lecture notes on finite group schemes.
Let $k$ be an algebraically closed field of characteristic $p$. The category of finite length $W(k)$-modules $N$ ...
3
votes
0
answers
733
views
When is a quotient by a strictly free group action étale?
Let $S$ be a base scheme and $G$ an $S$-group scheme acting on an $S$-scheme $X$ and assume that the quotient scheme $Y := X/G$ exists. (This is for example the case if $S$ is finite locally free and ...
3
votes
0
answers
109
views
G is p-divisible, about the affine rings of G[p]
Let $R$ be a ring and $G$ a $p$-divisible group over $R$. Since $G[p]$ is finite flat over $\text{Spec}(R)$, it is an affine (group) scheme, say $\text{Spec}(B)$. What can be said about $B$ beyond the ...
3
votes
0
answers
251
views
Schematic closure of maximal torus over a discrete valuation ring: smoothness of the special fibre
Let $R$ be a complete DVR (or just Henselian) with field of fractions
$F$ and algebraically closed residue field $k$ (of characteristic $p>0$). Let $G$ be a reductive group scheme over $R$ and let $...
3
votes
0
answers
221
views
A question on Kähler differentials and cotangent spaces on schemes
I have the following question (should be easy for those who know something about the field):
On page 92 (97 of the old edition) of Mumford's book "Abelian varieties", the author talks about an ...
3
votes
0
answers
917
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On the structure of commutative group schemes
The structure of a commutative affine algebraic group $G$ over a field $k$ is understood (SGA 3): $G$ has a maximal subgroup of multiplicative type $M$ and the quotient $G/M$ is unipotent.
I am ...
2
votes
0
answers
143
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Computing the Dieudonné module of $\mu_p$ from Fontaine's Witt Covector
In Groupes $p$-divisibles sur les corps locaux, Fontaine introduced a uniform construction of Dieudonné modules through the definition of the Witt covector. Consider a perfect field $k$ of ...
2
votes
0
answers
115
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Endomorphisms of the multiplicative group over a non-reduced complex analytic space
Let $S_n$ be the (usually) non-reduced complex analytic space corresponding to the ring $\mathbb{C}[X]/(X^{n+1})$ (the underlying topological space of $S_n$ is a point, and $S_0$ is a reduced point).
...
2
votes
0
answers
120
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"Vanishing locus" of forms in the $h$-topology
Let $\Omega_{h}^p$ be the sheaf of $p$-forms in the $h$-topology defined as the sheafification for the $h$-topology of the presheaf,
$$ Y \mapsto \Omega^p_Y(Y) $$
Kebekus and Schnell show that when $X$...
2
votes
0
answers
385
views
Connected-étale sequence $ 0 \to G^0 \to G \to G^{\text{ét}} \to 0$ for affine finite group scheme $G$
Let $G$ be an affine finite commutative group scheme over a complete (or at least Henselian; thanks to Jason Starr for calling attention to it) local ring $R$, and assume the residue field $\kappa=R/m$...
2
votes
0
answers
394
views
Finite Flat Group Scheme over a field $k$ of characteristic $0$ is always Etale
I have a question about a claim about classification of finite flat group schemes: https://en.wikipedia.org/wiki/Group_scheme#Finite_flat_group_schemes
A fin flat group scheme $G$ is of type $(a,b)$...
2
votes
0
answers
175
views
Is the Lie algebra of a flat group scheme still flat?
Let $\mathcal{O}$ be a discrete valuation ring, and $\mathcal{G}$ is a flat group scheme over $\mathcal{O}$, we may assume the generic fiber is reductive. Then can we define its Lie algebra, and ...
2
votes
0
answers
266
views
Étale group scheme exact sequence
Consider the exact sequence of finite flat group schemes over the $2$-adic integers ring $\mathbb{Z}_2$:
$$0\longrightarrow\mathbb{Z}/2\mathbb{Z}\longrightarrow A\longrightarrow\mathbb{Z}/2\mathbb{Z}\...
2
votes
0
answers
233
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Standard application of Oort-Tate classification theorem
$\DeclareMathOperator{\Spec}{Spec}
\DeclareMathOperator{\tors}{tors}$In Mazur's paper “Modular curves and the Eisenstein ideals”, on the bottom of page 159, it says that if $T$ is a open subscheme of $...
2
votes
0
answers
553
views
Tangent Space of Picard Scheme
Let $X$ be a scheme and it's Picard scheme $\underline{Pic}(X)$ exist. Denote by $\underline{Pic}^0(X)$ the connected component of the origin of
the Picard scheme. My question is what the geometric ...
2
votes
0
answers
211
views
Representability of a subfunctor of the functor of points of a group scheme
Let $G$ be a group scheme over a scheme $S$ and $h_G:\rm{Sch}/S\longrightarrow \rm{Grp}$ the functor of points represented by $G$.
Let $k$ be a subfunctor of $h$. Is $k$ representable? If so, can we ...
2
votes
0
answers
98
views
Classical reductive group schemes vs. unitary groups of separable algebras with involution --- reference request
Let $K$ be a field with $2\in K^\times$, let $A$ be a separable $K$-algebra (i.e. $A$ is finite-dimensional, semisimple and its center is an etale $K$-algebra), and let $\sigma:A\to A$ be a $K$-...