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Questions tagged [group-schemes]

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7
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0answers
114 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....
3
votes
1answer
175 views

Extend Group Action of $\mathbb{A}^1 /G$ to Projective Line

My question refers to an argument used in Torsten Ekedahl's paper: https://arxiv.org/abs/0903.3148 in Example ii) (page 8): We consider a finite subgroup of affine transformation of $\mathbb{A}^1$. ...
1
vote
0answers
149 views

Computing hom sheaf

Let $\mu_{m}$ and $\mu_{p}$ be the group scheme representing the roots of unity over a field $k$ of characteristic $p>0$. Assume that $p$ divides $m$. What does one compute the sheaf $\...
1
vote
0answers
95 views

first infinitesimal sheaf and canonical sheaf

Please prove this problem for me. Let $S$ be a scheme, $G$ be a group $S$-scheme and $\omega_{G} = e^{*}\Omega^{1}_{G/S} $ ($e: S \to G$ identity section). If the first infinitesimal neighborhood of ...
5
votes
1answer
353 views

What is $\mathrm{O}_q/\mathrm{SO}_q$ if $q$ is a quadratic $\mathbb{Z}$-form which is degenerate?

Any binary quadratic $\mathbb{Z}$-form $q$ induces a symmetric bilinear form $$ B_q(u,v) = q(u+v) - q(u) -q(v) \ \ \forall u,v \ \in\mathbb{Z}^2 $$ and it is considered non-degenerate (over $\mathbb{...
3
votes
2answers
176 views

Representability of Hom of two finite flat group schemes

I am reading a note at Page 63 ftp://ftp.math.ethz.ch/users/pink/FGS/CompleteNotes.pdf It says whenever $G$ is finite and flat over $S$ the functor ${\rm Hom}(G,H)$ is representable. But it does not ...
1
vote
0answers
126 views

Group scheme representation from action of a group scheme on a variety?

Let f be some homogenous polynomial of d. Let $X = \operatorname{Proj} (k[x,y,z]/(f))$ where $k$ is algebraically closed field of characteristic $p>0$. Now $G$ is a group scheme acting on $X$. ...
26
votes
1answer
776 views

Serre's remark on group algebras and related questions

I've recently heard about an idea of Serre that for each finite group $G$ there exists a group scheme $X$ such that for each field $K$ the group $X(K)$ is naturally isomorphic to the unit group of $K[...
2
votes
0answers
69 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 $K$-...
5
votes
0answers
193 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 ...
9
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0answers
107 views

Can elements in the orthogonal group of a non-split Azumaya algebra with an orthogonal involution have reduced norm -1?

Let $R$ be a connected (commutative) ring with $2\in R^\times$. Let $A$ be an Azumaya algebra over $R$ and let $\sigma:A\to A$ be an orthogonal involution. (This means that there is a faithfully flat ...
7
votes
1answer
325 views

Exponential map of a Formal Group Scheme

Let $k$ be a field of characteristic $0$ and let $\mathfrak{g}$ be a finite dimensional Lie algebra over $k$. $\mathfrak{g}$ corresponds to a formal group scheme $\mathcal{G} = \text{Spf} (U(\mathfrak{...
2
votes
0answers
71 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\...
2
votes
0answers
110 views

Representabillity of torsors and exact sequence of group schemes

I've seen the following property used at many places, I don't exactly why it is true. Let $S$ be a site with an initial object $e$. There is an exact sequence of group objects $$1 \to F \to G \to H \...
2
votes
0answers
116 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 ...
4
votes
1answer
405 views

What exactly is $\underset{n}{\varprojlim} \ \mu_{p^n}$?

Let $k$ be a field of characteristic $p> 0$. Let $\mu_{p^n}$ denote the group scheme $\mu_{p^n}(R) =\{ x \in R: x^{p^n}=1 \}$. Then there are natural maps from $\mu_{p^n}$ to $\mu_{p^{n-1}}$. I'm ...
7
votes
1answer
340 views

Example of a connected finite group scheme which is not solvable

What would be an example of a connected finite group scheme over a field $k$ that is not solvable? Here $k$ is algebraically closed. Let $\operatorname{GL}_n$ be the general linear group scheme over ...
2
votes
0answers
72 views

Isomorphism between Group Schemes over $\mathbb{Z}_2$

Consider the Matrix $$M:= \left(\begin{matrix} 1 & 0 \\ 0 & d \end{matrix}\right) $$ for an odd $d \in \mathbb{Z}$. Define the group scheme $Sp(g)$ defined over $\mathbb{Z}$ with ...
2
votes
1answer
153 views

How can we view the local Weil group as a group scheme over $\mathbf Q$?

Let $F$ be a $p$-adic local field with residue field $\kappa$, and $q = |\kappa|$. Then the residue field $\overline{\kappa}$ of $F^{\textrm{ur}}$ is an algebraic closure of $\kappa$. The local ...
11
votes
0answers
206 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 ...
3
votes
0answers
234 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
1answer
174 views

Torsors of pushforward group schemes

I'm reading William Waterhouse's "Discriminants of etale algebras and related structures", and he makes a basic claim I'm struggling to justify. Suppose $S/R$ is etale of rank $n$... and let $\pi$ ...
3
votes
1answer
394 views

Representation of a group scheme

Let $\mathcal{G}$ be a affine algebraic group scheme(may not be reductive) over a scheme $S$. How to define a rational representaion of $\mathcal{G}$ (over $S$)? Is there always a faithful ...
1
vote
0answers
117 views

Convolution of $\ell$-adic sheaves and group homomorphisms

This question follows this one , where I defined convolution of $\ell$-adic/perverse sheaves. Here I am working with a perfect field $k$ ($char(k)\neq l$) and with a smooth separated groupscheme $G$ ...
2
votes
0answers
191 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 ...
2
votes
0answers
131 views

understanding full set of sections as in katz-mazur

I was reading this question, specifically Brian's answer. In particular I am having a bit of trouble digesting the following sentence: Being a "full set of sections" of $Z/S$ is something which is ...
2
votes
0answers
96 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
0answers
185 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 $...
7
votes
0answers
163 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 $S$-...
2
votes
0answers
40 views

Special unitary group of an affine algebra is integral

Let $R$ be an affine $\mathbb C-$algebra with a linear involution $x\rightarrow \bar x=\iota(x)$, let $S=R/\iota$ and $\psi:R^n\times R^n\rightarrow R$ be an $R/S-$hermitian form. Finaly let $$SU_n(R)=...
2
votes
1answer
270 views

Lie algebra of affine group scheme

Let $G$ be an affine group scheme over a characteristic zero field $k$ (the case I have in mind is $k=\mathbb{Q}$). Let $H$ be a subgroup of $G$ which satisfies $\mathrm{Lie}(H)=\mathrm{Lie}(G)$. Is ...
9
votes
1answer
358 views

Is $G$ always the automorphism group of the trivial $G$-torsor?

If $G$ is just an ordinary set-theoretic group, then the answer to the question in the title is yes: the automorphisms of $G$ as a (left) $G$-set are all of the form "multiply (on the right) by an ...
2
votes
1answer
300 views

Birational Group Law

Let $S$ be a scheme and $X$ a smooth separated faithfully flat over $S$. An $S$-birational group law on $X$ is an $S$-rational map $$m:X\times_S X\dashrightarrow X, (x,y)\mapsto xy$$ such that ...
2
votes
1answer
222 views

Derived Group Scheme

If $G$ is a $k-$ group scheme (seeing as a functor) exist a good definition of what is the derived group scheme? (Or a good reference for a good definition). Where derived I'm talking in the sense of ...
3
votes
2answers
319 views

Do all reductive group schemes over semilocal rings admit finite-dimensional free faithful representations?

The definition of a reductive group scheme is as in SGA III. Frankly, I only know that they exist for the adjoint group (the adjoint representation). In SGA III, I could only find a result for general ...
4
votes
1answer
185 views

Flatness of Weil restriction

Let $X\rightarrow Y$ a ramified double cover of smooth projective curves, and let $$\mathcal G:=Res_{X/Y}(SL_n)$$ be the Weil restriction of the constant group scheme $SL_n$ over $X$. Question: Is ...
3
votes
1answer
130 views

Maximality of connected components of finite flat group schemes

Let $k$ be a perfect field of characteristic $p>0$. Let $K$ be a finite, totally ramified extension of $K_0:=\mathrm{Frac}\ W(k)$ and let $\mathcal{O}_K$ be the ring of integers of $K$. All group ...
3
votes
1answer
213 views

Chevalley devissage

Let $G$ be an algebraic group over a perfect field $k$. Then it is know that it can be written as an extension of an affine algebraic group and a proper algebraic group. Is there a similar result for ...
5
votes
1answer
458 views

Lie algebra and base change

I am very confused by the following and would appreciate any help. Let $\mu_p \subset \mathbb{G}_m$ be the $p$-torsion subgroup scheme of the multiplicative group over $\mathbb{Z}_p$. I would like to ...
3
votes
2answers
187 views

Reference request: groups of multiplicative type are closed under extensions

I remember reading (quite a while ago, and I can't remember where!) that linear algebraic groups of multiplicative type over a field of characteristic zero are closed under extensions. This is ...
9
votes
1answer
577 views

Is there a non-smooth algebraic group scheme in char $p$, all of whose defining relations have degree less than $p$?

Let $k$ be an algebraically closed field of characteristic $p>0$. All the examples of non-smooth algebraic group schemes over $k$ that I have seen (apart from "artificial" examples; see below) have ...
3
votes
1answer
134 views

Approximation of Group Schemes over valuation rings

In his paper Approximation des schémas en groupes, quasi compacts sur un corps, Daniel Perrin shows that every quasi compact group scheme over a field then it is an inverse limit of group schemes of ...
2
votes
0answers
182 views

From algebraic group actions to group scheme actions

I am trying to understand the basic results of geometric invariant theory. I want to pull off the band aid and use Mumford, but am a neophyte with respect to scheme theory. Thus, I have been trying to ...
2
votes
0answers
89 views

split tori over local fields

Let $F$ be a non-archimedean local field, and $\mathscr O$ its ring of integers. Suppose $T$ is an $F$-split torus, i.e., $T = (\mathbb G_m)^r$ where $\mathbb G_m$ denotes the multiplicative group. ...
4
votes
1answer
246 views

Normal Hopf ideals and Hopf subalgebras, quotient group schemes

There is a following theorem: $H$ is a commutative Hopf algebra over a field $k$. Then there exists a bijective correspondence between $$\{ \textrm{Hopf subalgebras }K\subset H \} \quad \...
0
votes
0answers
99 views

Lifting points of étale group scheme

Consider the following setting: let $(R,\mathfrak{m})$ be a Noetherian complete discrete valuation ring (with maximal ideal $\mathfrak{m}$) and let $K$ be its field of fractions. Now take $L$ be the ...
8
votes
1answer
483 views

Group schemes, adeles, double cosets, and étale cohomology

Let $K$ be a number field, $R$ the ring of integers of $K$, ${\mathbf{A}^f}$ the ring finite adeles of $K$, and ${\widehat{R}}\subset {\mathbf{A}^f}$ the ring of integral adeles. Let $G$ be an affine ...
13
votes
2answers
478 views

Infinitesimal deformations of the formal group of $\mathbb{G}_m$

For a commutative ring $R$, consider the formal group $\widehat{\mathbb{G}}_m$ over $R$ that is the completion of $\mathbb{G}_{m, R}$ along its identity section (naively, $\widehat{\mathbb{G}}_m$ is ...
1
vote
0answers
116 views

Condition for a finite group scheme to be étale [closed]

My question comes from the reading of Tate's paper $p$-divisible groups. In the last few pages there is an argument which gives as trivial the following fact. If we take a $p$-divisible group over a ...
4
votes
0answers
278 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_{\...