Questions tagged [profinite-groups]
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310
questions
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On the conjugation action on the first congruence subgroup of special linear group over $p$-adic fields
Let $\mathcal{O}_{L}$ be the ring of integers of a finite extension $L$ of $p$-adic number fields $\mathbb{Q}_{p}$ where $p$ is an odd prime. Let $\mathfrak{m}_{L}$ be the maximal ideal of $\mathcal{O}...
4
votes
1
answer
453
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Subgroup of p-adic units
Let $\left(\widehat{\mathbb Z}\right)^\times=\prod_p{\mathbb Z}_p^\times$
be the unit group of the ring $\widehat{\mathbb{Z}}$, which is the profinite completion of $\mathbb Z$.
We give it the product ...
3
votes
0
answers
273
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Semidirect product in inverse Galois problem
Let $L/\mathbb{Q}$ (resp. $K/\mathbb{Q}$) be a Galois extension of rational number field $\mathbb{Q}$ with Galois group $P$ (resp. $H$) where $P$ is a second countable pro-$p$ group and $H$ is a ...
4
votes
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104
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Hecke algebra $\mathcal{H}(K_1\backslash \mathrm{GL}_n(\mathbb{F})/K_1)$
$\DeclareMathOperator\GL{GL}$Let $\mathbb{F}$ be a non-Archimedean local field. Let $\mathcal{O}$ be its ring of integers, and let $\frak{m}$ be its maximal ideal Let $\GL_n(\mathcal{O})$ be the group ...
1
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0
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56
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Matroid for Laurent series
I am trying to find a matroid for profinite rings which are the inverse limit of their finite quotients, and whose linearly independent elements are of the form $L((t_1,\dots,t_n))$.
To set this up, ...
1
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0
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45
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Sylow subgroups of the free product of profinite groups
I am interested in the Sylow subgroups of the profinite completion of a free product of finite groups.
Is the following naive expectation true ? I assume things like this should be well-known, and am ...
3
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61
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Metrisable profinite groups
I do not understand on page 6 of Galois Cohomology from Serre, the comment after exercise 2) part d). He claims that taking G to be the dual of a countably dimensional vector space over $\mathbb{F}_p$ ...
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122
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Finitely generated torsion-free pro-$p$ subgroup of ${\rm GL}_{n}(\mathbb{F}_{p}[[T]])$ is solvable?
Let $\mathbb{F}_{p}$ be a finite field of order $p$, and $\mathbb{F}_{p}[[T]]$ be the ring of formal power series over $\mathbb{F}_{p}$. My question is the following:
Let $G$ be a closed pro-$p$ ...
2
votes
0
answers
89
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Hereditarily just-infinite pro-$2$ groups
An infinite profinite group $G$ is called just-infinite if all non-trivial closed normal subgroups of $G$ have finite index. A profinite group is called hereditarily just-infinite if every open ...
1
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0
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118
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$p'$-automorphisms of pro-$p$ groups
Let $p$ be a prime and $G$ be a finitely generated pro-$p$ group admitting a continuous automorphism $\phi$ of finite order relatively prime to $p$. Let $\Phi(G)$ denote the Frattini subgroup of $G$. ...
1
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0
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76
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Non-Noetherian closed subgroups of ${\rm GL}_{n}(\mathbb{F}_{q}[[T]])$
Let $\mathbb{F}_{q}$ be a finite field of order $q$, and $\mathbb{F}_{q}[[T]]$ be the ring of formal power series over $\mathbb{F}_{q}$. We say that a profinite group $G$ is Noetherian if any closed ...
2
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137
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Prime-to-$p$ quotients of ${\rm PSL}_{2}(\mathbb{Z}_{p})$
Let $p$ be a prime and $\mathbb{Z}_p$ the ring of $p$-adic integers. Let ${\rm PSL}_{2}(\mathbb{Z}_{p})={\rm SL}_{2}(\mathbb{Z}_{p})/\{\pm 1\}$ be the projective special linear group over $\mathbb{Z}...
2
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92
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If $F$ is a prosoluble subgroup of a free profinite product $\amalg G_i$ and $F \cap G_i^g$ is pro-$p$, is also $F$ pro-$p$?
There is a 1995 paper (Manusc. Math., DOI link) of Florian Pop where he proves the following:
Theorem. Let $G$ be the free product of profinite groups $G_i$ and $F$ a closed prosoluble subgroup of $G$...
3
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0
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148
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When is a group the same as its profinite completion
I'm working with the inner automorphism group of profinite quandles. A question I have yet to resolve is whether or not the inner automorphism group of a profinite quandle is necessarily profinite, or ...
8
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answer
160
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Stone-topological/profinite equivalence for quandles
A quandle $(Q,\triangleleft,\triangleleft^{-1})$ is a set $Q$ with two binary operations $\triangleleft,\triangleleft^{-1}:Q\times Q\to Q$ such that the following hold for all $x,y,z\in Q$:
(Q1) ...
6
votes
2
answers
184
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Agemo-of-agemo inclusions for p-groups
For a finite $p$-group $G$, let $\mho_i(G)$ denote the subgroup generated by $p^i$-powers of elements of $G$.
It is well-known that $\mho_i(\mho_j(G))$ can differ from $\mho_j(\mho_i(G))$ and from $\...
4
votes
1
answer
192
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Profinite groups with isomorphic proper, dense subgroups are isomorphic
I am developing a sort of standard representation for profinite quandles. This involves profinite groups a lot, actually. In one part of my construction the filtered diagram used to construct a ...
4
votes
0
answers
162
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The order of the global Galois group
For any profinite group $G$, we can define the order of $G$ using the notion of supernatural number. Now let $K$ be a number field, $S$ a finite set of primes of $K$ and $ G_{K,S} $ the Galois group ...
1
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88
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Existence of countable dense normal subgroups of global Galois group
Let $K$ be a number field, $S$ a finite set of primes of $K$ and $ G_{K,S} $ the Galois group of the maximal extension of $ K $ (inside a fixed algebraic closure of $K$) unramified outside $ S $. In ...
0
votes
0
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49
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Existence of maximal topologically characteristic subgroup of infinite index of pro-$p$ groups
Let $G$ be a topologically finitely generated infinite pro-$p$ group. Suppose that $G$ is not just-infinite. Does the group $G$ always have a maximal topologically characteristic subgroup of infinite ...
7
votes
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310
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Topological generators for $\mathrm{SL}_2(\mathbf{Z}_p)$
$\DeclareMathOperator\SL{SL}$ Let $p>3$ and $G$ be an open subgroup of the special linear group $\SL_2(\mathbf{Z}_p)$ over the ring $\mathbf{Z}_p$ of $p$-adic integers. Suppose that $G$ is ...
0
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0
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76
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Finite pro-$ p $ subgroups of $ {\rm SL}_{2}(\mathbb{F}[[T]]) $
Let $ p $ be an odd prime, $ \mathbb{F} $ a finite field of characterisitc $ p $ and $ \mathbb{F}[[T]] $ the formal power series over $ \mathbb{F} $. Let $ G $ be a pro-$ p $ subgroup of $ {\rm SL}_{2}...
0
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1
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147
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Topological generators for the Sylow pro-$p$ subgroup of $\mathrm{SL}_2(\mathbf{Z}_p)$
$\DeclareMathOperator\SL{SL}\DeclareMathOperator\GL{GL}$Let $G_2(\mathbf{Z}_p):=\begin{pmatrix}
1+p\mathbf{Z}_{p} & \mathbf{Z}_{p}\\
p\mathbf{Z}_{p} & 1+p \mathbf{Z}_{p}
\end{pmatrix}$. ...
1
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0
answers
86
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A closed subgroup of $p$-adic analytic group having same dimension is open?
Let $G$ be $p$-adic analytic pro-$p$ group and $H$ a closed subgroup of $G$. Suppose that $G$ and $H$ have the same dimension as $p$-adic analytic groups.
Question: Is it true that $H$ is an open ...
3
votes
1
answer
167
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Open conjugacy classes in a second countable profinite group
Let $G$ be a second countable profinite group, $g\in G$ and $g^G:=\{hgh^{-1}~|~h\in G\}$ the conjugacy class of $g$ in $G$. Theorem 3.2 in Wesolek's Conjugacy class conditions in locally compact ...
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115
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Multiplicativity of Euler–Poincaré characteristics of cohomology of pro-$p$ groups
While reading a paper, I found a mentioning that for an extension $1 \rightarrow H \rightarrow G \rightarrow N \rightarrow 1$ of pro-$p$ groups, the Euler–Poincaré characteristics $\chi(H)$, $\chi(G)$,...
2
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0
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56
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Convergent condition in the definition of free profinite groups
I have a (perhaps simple) question about free pro-$C$ constructions.
Definition. Let $X$ be a set. The free pro-$C$ group over $X$ is a pro-$C$ group $F$ together a $1$-convergent map $f: X \to F$ ...
2
votes
0
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168
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When is an infinite pro-$p$ group generated by its torsions
Let $p$ be a prime and $\mathcal{O}=\mathbf{Z}_p$ or $\mathbf{F}_p[[T]]$, i.e. the ring of $p$-adic integers or the ring of formal power series over a finite field $\mathbf{F}_p$ of order $p$. Let $G\...
3
votes
1
answer
142
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Profinite completion of Baumslag-Solitar group as a profinite HNN-extension
I apologize if this question is basic in some sense. I was looking for an example of a non-proper HNN-extension and I found this.
In the comments, markvs mentioned the Baumslag-Solitar group $B(2,3)$. ...
3
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0
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130
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Just-infinite quotients of pro-$p$ groups that are linear over a complete Noetherian local ring
This question is a sequel to Quotients of pro-p
groups linear over a complete Noetherian local ring. Recall that an infinite pro-$p$ group is called just-infinite if it has no proper, infinite ...
2
votes
1
answer
242
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Is the free profinite group (or pro-$p$) torsion-free?
Let $X$ be a set and $\widehat{F}(X)$ the restricted free profinite group on $X$. To get $\widehat{F}(X)$ we define a profinite topology on $F$ (the free abstract group on $X$) and take $\widehat{F}(X)...
5
votes
1
answer
406
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“Sheaf cohomology” of Galois groups
Apparently the Galois group $G$ of a Galois extension $E/F$ can be viewed as an “étale sheaf” on the set $X$ of intermediate Galois extensions equipped with an appropriate Grothendieck topology (see ...
3
votes
0
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238
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Is insoluble $p$-adic analytic just-infinite pro-$p$ group torsion-free?
Recall that an infinite pro-$p$ group $G$ is called just-infinite if all non-trivial closed normal subgroup of $G$ have finite index.
Question: Let $G$ be an insoluble $p$-adic analytic just-infinite ...
1
vote
1
answer
89
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Quotients of pro-$p$ groups linear over a complete Noetherian local ring
Let $R$ be complete Noetherian local ring with finite residue field $\mathbb{F}$ of characteristic $ p $. We say that a pro-$p$ group $G$ is linear over $R$ if it is isomorphic to a closed subgroup of ...
2
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1
answer
226
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For locally profinite groups $H\lhd G$, is there a spectral sequence $\newcommand\@[2]{{\rm Ext}_#1^{#2}(\pi_1,\pi_2)}H^p(G/H,\@Hq)\implies\@G{p+q}$?
Let $G$ be a locally profinite group and let $H$ be a closed normal subgroup. Let $\pi_1$ and $\pi_2$ be two smooth complex representations of $G$. Is there always a spectral sequence as follows?
$$...
2
votes
1
answer
198
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Finite quotients of $p$-adic congruence subgroups of $\operatorname{SL}_2$
$\DeclareMathOperator\SL{SL}$Let $p$ be an odd prime and let $ \SL^1_2(\mathbb{Z}_p)$ denote the kernel of the natrual surjective morphism $\SL_2(\mathbb{Z}_p)\rightarrow \SL_2(\mathbb{Z}_p/p\mathbb{...
8
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276
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Quantizing the size of a pro-$p$ group
Let $p$ be a prime number and $G$ be a pro-$p$ group (not necessarily powerful). Let $\Omega$ denote the completed group algebra $\mathbb{F}_p[[G]]:=\varprojlim_N \mathbb{F}_p[G/N]$, where $N$ ranges ...
2
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0
answers
224
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Structure theorem for finitely generated profinite abelian groups
Is there a structure theorem for finitely generated profinite abelian group like a structure theorem of f.g. abelian group?
2
votes
1
answer
129
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The dimension of a torsion-free $p$-adic analytic group generated by two generators
$\DeclareMathOperator\GL{GL}$Let $G$ be a $2$-generator pro-$p$-group of finite rank, i.e. it is isomorphic to a closed subgroup of $\GL_d(\mathbb{Z}_p)$ for some integer $d$. Assume that $G$ is ...
5
votes
1
answer
413
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Classification of natural endomorphisms on finite groups
Any $z \in \widehat{\mathbb{Z}} = \lim_{n} \mathbb{Z}/n\mathbb{Z}$ defines an operation on all finite groups: if $G$ is a finite group and $g \in G$, say $g^n=1$, then map it to $g^{z_n}$. This ...
7
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1
answer
247
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When a pro-$p$ group of finite rank can be embedded into the first congruence subgroup of ${\rm GL}_{N}(\mathbb{Z}_{p})$
$\DeclareMathOperator\GL{GL}\DeclareMathOperator\SL{SL}$Let $p$ be a odd prime. We say that a pro-$p$ group has finite rank if it is isomorphic to a closed subgroup of $\GL_d(\mathbb{Z}_p)$ for some ...
1
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0
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148
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Generators of of $p$-adic congruence subgroups of $\operatorname{SL}_2$
$\DeclareMathOperator\SL{SL}$Let $p$ be an odd prime, $i$ a fixed positive integer and let $\Gamma_i$ denote the kernel of the map $\SL_2(\mathbb{Z}_p)\rightarrow \SL_2(\mathbb{Z}_p/p^i\mathbb{Z}_p)$. ...
2
votes
1
answer
140
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Nontrivial abelianization of torsion-free pro-$p$-group which contains a dense free subgroup is infinite?
Let $G$ be a topologically finitely generated pro-$p$-group. Assume that $G$ is torsion-free, it contains a dense free subgroup of infinite rank and the (topological) abelianization $G^{\text{ab}}$ of ...
1
vote
2
answers
155
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Looking for an example of profinite groups
Is there a profinite group $G$ with a locally finite subgroup $H$ such that $\overline H$, the closure of $H$, is not torsion?
0
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76
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A question about a class of pro-$\mathcal X$-group
This question concerns the following lemma of this paper:
Lemma 2. Let $\mathcal X_1,\ldots,\mathcal X_n$ be classes of finite groups closed with respect to normal subgroups and subdirect
products ...
1
vote
1
answer
155
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Openness of product of two open subgroups
Let $G$ be a profinite topological group with two closed subgroup $G_1$ and $G_2$. Suppose $G_1$ is normal in $G$ and $G=G_1G_2$. Let $H_i$ be an open subgroup in $G_i$ for $i=1,2$.
Question: Is $ ...
7
votes
0
answers
113
views
Endo reversible words
Let $w$ be a word in free group $F$ on finitely many generators. We will look at $w$ as word map on groups. It is clear that there exists an endomorphism $\phi$ of $F$ such that $\phi(w) = w^{-1}$ if ...
2
votes
1
answer
135
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Subgroup growth of direct product
I have started reading about subgroup growth and, to my surprise, I haven't found a reference to whether direct products preserve subgroup growth.
Recall that, given a finitely generated group $G$, ...
5
votes
1
answer
650
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Structure of a profinite group as a condensed set with an action of an open subgroup
Let $G$ be a profinite group and $H$ be an open subgroup. As a continuous $H$-topological space, we have $G=\coprod_{G/H} H$. Does this also hold as condensed sets, i.e. do we have an identification ...
1
vote
0
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122
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An infinite profinite group such that any $\overline{\mathbb{F}_{p}((t))}$-adic representation has finite image
This question is a sequel to An infinite profinite group such that any $p$-adic representation has finite image
.
Fix a prime $ p $. We call an infinite profinite group $G$ a Boston group (with ...