Questions tagged [finite-groups]

Questions on group theory which concern finite groups.

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-1 votes
0 answers
36 views

Some special subgroups of nilpotent groups of nilpotency class 2 [closed]

Let $G$ be a group. Denote by $\mathrm{Z}(G)$ and $G'$ the center of $G$ and the derived subgroup of $G$, respectively. Assume $G'\subseteq\mathrm{Z}(G)$. Then, it is clear that $G$ is a nilpotent ...
-4 votes
0 answers
67 views

Direct Product of Finite Groups [closed]

Recently, I am try to solve a problem in character theory: Character extension about $Q_8$ In this problem we have that $G=G/G'\cap N\lesssim G/G'\times G/N=G/G'\times Q_8$. If $G=N\times Q_8$, then $\...
5 votes
1 answer
109 views

How to classify homomorphisms from $\operatorname{PSL}(2,p)$ to $\operatorname{PGL}(n,2)$ when $2^n=p+1$?

$\DeclareMathOperator\PSL{PSL}\DeclareMathOperator\Aut{Aut}\DeclareMathOperator\PGL{PGL}\DeclareMathOperator\GL{GL}$I am looking to classify the homomorphisms from the group $\PSL(2,p)=\Aut(\mathbb{P}^...
3 votes
1 answer
113 views

Normalisers and stabilisers in classical groups $\operatorname{PGL}_{4}$

In $G=\operatorname{PGL}(4,5)$ there are two elementary abelian $2$-subgroups of order $16$ denoted by $E_{1}$ and $E_{2}$ with $N_{G}(E_{1})=E_{1}.\operatorname{Sp}(4,2)$ and $N_{G}(E_{2})=E_{2}.(2^{...
1 vote
0 answers
98 views

The property of self-normalizing subgroup

$G$ is a finite solvable group. Let $\{P_{1}, P_{2}, \dotsc , P_{s}\}$ be a Sylow basis of $G$. We have that $G=P_{1}P_{2}\dotsm P_{s}$. Set \begin{equation} \begin{aligned} %% The alignment is ...
2 votes
0 answers
69 views

Suzuki-Ree Lie algebras

Do the Suzuki and Ree groups of Lie type have associated Lie algebras over finite fields in the same way that the other groups of Lie type do? These algebras would be 5-dimensional over $\mathbb{F}_{2^...
5 votes
1 answer
374 views

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 ...
0 votes
0 answers
90 views

$G=HK$, $N_H(P_4)=P_4$ and $N_K(P_4)=P_4$

$G$ is a solvable group. Let $\pi(G)=\{p_{1}, p_{2}, p_{3}, p_{4}\}$ and $\{P_{1}, P_{2}, P_{3}, P_{4}\}$ be a Sylow basis of $G$. We have that $G=P_{1}P_{2}P_{3}P_{4}$. Set $H=P_{1}P_{2}P_{4}$ and $K=...
1 vote
0 answers
145 views

Character extension about $Q_8$

Recently, I am studying the book Navarro - Character Theory and the McKay Conjecture. I am trying to solve the following exercise: (Exercise 5.9) Let $G$ be a finite group and $N\unlhd G$, suppose ...
-3 votes
1 answer
114 views

Exponential order of unipotent elements in an endomorphism ring of abelian groups

$\DeclareMathOperator\End{End}\newcommand{\Id}{\mathrm{Id}}$Let $E=\End(I)$ be the endomorphism ring of the abelian group $I$. We have the following statement for $B\in E$, $p$ a prime number and $r$ ...
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1 vote
1 answer
190 views

Kronecker product preserves the conjugacy relation?

Let $G =$ PGL$_{n}(\textbf{C})$ and $T$ be the image in $G$ of the subgroup of the invertible diagonal matrices of $\operatorname{GL}_{n}(\textbf{C})$. Let $A$ and $B$ be two elementary abelian $2$-...
6 votes
0 answers
261 views

(CFSG-free) Finite simple groups whose character degrees square divide its order

Let $G$ be a finite group. It is well-known that for all irreducible complex character $\chi$ then $\deg(\chi)$ divides $\lvert G\rvert$. Motivated by some problems with modular tensor categories, we ...
1 vote
0 answers
207 views

Presentation complexes with same homology and different fundamental groups

If we start with a perfect group $G$ of deficiency zero then there is a presentation $P$ of $G$ such that the number of relations and the number of generators for $P$ are the same. For such $P$, the ...
  • 159
4 votes
1 answer
209 views

Does a perfect $4^{11}\cdot M_{24}$ exist?

Is there any perfect group which could be notated as $4^{11}\cdot M_{24}$ (a non-split extension of the largest Mathieu group by a homocyclic group of type $4^{11}$)?
3 votes
0 answers
46 views

Hamilton cycles in Cayley graphs: between Rapaport-Strasser and Fleischner

A well-known question of Rapaport-Strasser asks whether every finite connected Cayley graph has a Hamilton cycle. Fleischner's Theorem implies that if $S$ is the generating set of such a Cayley graph $...
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1 vote
0 answers
101 views

Embedding (Kronecker product) preserves the structure?

In PGL$_{n}(\textbf{C})$, conjugacy classes of toral involutions can be represented by $$s_{i} = \begin{pmatrix} -I_{i} & 0\\ 0 & I_{n-i} \end{pmatrix}.$$ for $1 \leqslant i \leqslant [n/2]$. ...
2 votes
0 answers
109 views

Homogeneity of finite simple groups

According to A complete classification of finite homogeneous groups, the finite simple groups $G$ which are homogeneous (meaning every isomorphism between subgroups extends to an automorphism of $G$) ...
7 votes
0 answers
191 views

Locally finite groups containing all finite groups

Say that a group is rich if is contains isomorphic copies of all finite groups. It is easy to produce rich groups, and also rich locally finite groups, for instance the restricted direct product $A=\...
  • 54k
2 votes
1 answer
166 views

Invariants of the group algebra of a finite group

Consider a finite group $G$ and its complex group algebra $V_G$, on which $G$ acts. I would like to know: what are the polynomial $G$-invariants of $V_G$ i.e., the polynomial functions $p\in \mathbb{C}...
0 votes
1 answer
109 views

Intersection of identity components

Let $e_{1}$ and $e_{2}$ be involutions in the algebraic group $G=\operatorname{PGL}_{n}(\mathbb{C})$. Do we have $$C_{G}(\langle e_{1},e_{2}\rangle)^{\circ} = C_{G}(e_{1})^{\circ}\cap C_{G}(e_{2})^{\...
1 vote
0 answers
51 views

Centralisers of involutions not quasi-isolated

The quasi-isolated elements of the algebraic group $G=\operatorname{PGL}_{n}(\mathbb{C})$ are classified in the paper "quasi-isolated elements in reductive groups" by C. Bonnafe. Let's focus ...
4 votes
1 answer
190 views

Wedderburn decomposition of special linear groups

$\DeclareMathOperator\SL{SL}\newcommand\card[1]{\lvert#1\rvert}$I want to study about Wedderburn decomposition of group algebra $k\SL(n,\mathbb{F}_p)$ where $k$ is either an algebraically closed field ...
11 votes
2 answers
309 views

Minimal irrep of $\mathrm{PSL}(2,p) $

$\DeclareMathOperator\PSL{PSL}\DeclareMathOperator\SL{SL}\DeclareMathOperator\SO{SO}\DeclareMathOperator\SU{SU}$Let $ p $ be a prime for which $ \PSL(2,p) $ is simple (so $ p \neq 2,3 $). Is the ...
0 votes
0 answers
152 views

Signed permutations and $ \operatorname{SO}(n) $

$\DeclareMathOperator\SO{SO}\DeclareMathOperator\O{O}\DeclareMathOperator\SU{SU}\DeclareMathOperator\Lift{Lift}$The subgroup of $ \SO(n) $ of determinant-$1$ signed permutations has order $ n!2^n/2 $. ...
1 vote
1 answer
80 views

Cohomological variety in case that Sylow subgroup is elementary abelian

Let $G$ be a finite group, $p$ a prime number, and $k$ an algebraically closed field of characteristic $p$. Then we can consider the cohomological variety of $G$, namely the maximal spectrum $V_G$ of ...
2 votes
0 answers
57 views

Presentation complex and arbitrary $2$-dimensional CW-complex with same fundamental group

Given a finite group $G$, consider a presentation $P$ of $G$ and consider $X_P$, the presentation complex. Now let $Y$ be any $2$-dimensional CW-complex with $\pi_1(Y)=G$. Is there any relation ...
  • 159
1 vote
0 answers
209 views

Could there be a better classification of finite simple groups?

The current classification of finite simple groups puts every finite simiple group in one of a few categories. There are the "nicely" behaved infinite categories (cyclic, alternating, Lie-...
  • 419
1 vote
0 answers
90 views

Irreducibility of adjoint representation

Let $ \mathbb{F} $ be a finite field of characteristic $ p\geq 5 $, $ G $ a finite group and $ \rho:G\to {\rm GL}_{2}(\mathbb{F}) $ be a representation of $ G $. By $ \text{ad}^{0}(\rho) $ we denote ...
  • 491
5 votes
0 answers
160 views

Finite groups with number of generators strictly less than number of relations

For the finite cyclic group of order $n$, there is the standard presentation $\langle a \mid a^n\rangle$. Also for $S_n$ (symmetric group), I know a few presentations where the number of relations is ...
  • 159
0 votes
0 answers
74 views

A decision problem of an inverse problem in finite group theory

A finite group $G$ is called integral if there is a finite group $H$ such that $G\cong H'$. In Araujo, Cameron, Casolo, Matucci's paper, integrals of groups, they tried to solve a problem as following:...
1 vote
0 answers
127 views

Finite simple groups of order $p+1$

$\DeclareMathOperator\PSL{PSL}\DeclareMathOperator\PSU{PSU}$Cross-post from MSE. There are some very interesting comments on the original post if you want to go check it out. Are there any well known ...
3 votes
1 answer
159 views

Extensions of a simple group by an elementary abelian $p$-group

Let $V$ be an elementary abelian $p$-group of size $p^n$. Let $G$ be a finite group with $V\unlhd G$ such that $G/V=H$ is simple (like $\operatorname{PSL}(m,q)$ with $q$ a power of $p$ or any other ...
5 votes
1 answer
258 views

How to make Burnside's formula compatible with point counting for varieties over finite fields?

If $G$ is a finite group acting on a finite set $X$, we have Burnside's formula that counts the number of orbits $|X/G|$ as: $$ |X/G| = \frac1{|G|} \sum_{g\in G} |X^g|, $$ with $X^g$ being the set of ...
1 vote
0 answers
73 views

Second homology group of a presentation complex

I am trying to learn results related to the presentation complex of a group and I am new to this subject. So I apologize if the questions are silly. Given a finite group $G$, and a presentation $P$ of ...
  • 159
14 votes
0 answers
301 views

Is this class of groups already in the literature or specified by standard conditions?

In recent work Lifting $N_\infty$ operads from conjugacy data on homotopical combinatorics / $N_\infty$ operads in equivariant homotopy theory, collaborators Scott Balchin, Ethan MacBrough, and I ...
7 votes
0 answers
99 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 ...
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7 votes
1 answer
254 views

"Novelty" maximal subgroups in $S_n$

What are the maximal subgroups $M < S_n$ such that $M \cap A_n$ is not maximal in $A_n$? Maximal subgroups of $S_n$ are described by the O'Nan-Scott theorem and very extensively studied in many ...
  • 2,576
4 votes
0 answers
197 views

Finite 2-groups with $(ab)^{2}=(ba)^{2}$

There exist nonabelian finite 2-groups $G$ with the property $(A2)$ : for every $a,b\in G$, $(ab)^{2}=(ba)^{2}$. An example of a such group is given by the quaternion group $Q_{8}$ of order 8. Is ...
  • 913
0 votes
0 answers
189 views

Groups of orders $7!$ and $\frac{7!}{2}$

In our research, we need to know that whether every group $G$ of order $2520 = 2^3 \cdot 3^2 \cdot 5 \cdot 7=\frac{7!}{2}$ or $5040 = 2^4 \cdot 3^2 \cdot 5 \cdot 7=7!$ has a proper subgroup non-...
-1 votes
2 answers
236 views

Splitting of a finite group with no abelian subfactor in composition series

Let $G$ be a finite group with no abelian subfactor in its composition series. Is $G$ obtained from simple groups by iterating semidirect products? (Initially it was asked whether $G$ is a direct ...
  • 151
4 votes
1 answer
296 views

Converse of Clifford's theorem for a semidirect product

Suppose that a group $G$ is a semidirect product $G = N \rtimes H$ with $N \trianglelefteq G$. Let $\mathbb{F}$ be a field. Say $V$ is a finite-dimensional $\mathbb{F}[G]$-module such that $V \...
  • 2,576
5 votes
1 answer
346 views

The number of polynomials on a finite group, II

This question is follow up of this MO-post. First let us recall the necessary definitions. A function $f:X\to X$ on a group $X$ is called a polynomial if there exists $n\in\mathbb N$ and elements $a_0,...
  • 35.5k
2 votes
0 answers
44 views

Finite groups whose polynomials share two common properties with polynomials on commutative groups

This question is motivated by (some available information on) this MO-problem on the largest possible degree of a polynomial on a finite group and this MO-problem on the degree of the constant ...
  • 35.5k
9 votes
1 answer
452 views

The degree of a constant polynomial on a finite group

A function $f:X\to X$ on a group $X$ is called a polynomial if there exists $n\in\mathbb N=\{1,2,\dots\}$ and elements $a_0,a_1,\dots,a_n\in X$ such that $f(x)=a_0xa_1x\cdots xa_n$ for all $x\in X$. ...
  • 35.5k
4 votes
1 answer
169 views

Prime divisors of nonabelian simple group and of its outer automorphism group

Let $G$ be a finite nonabelian simple group. Write $\mathrm{Out}(G)$ the outer automorphism group of $G$. For a finite group $H$, let $\pi(H)$ be the prime divisors of the order of $H$. By check the ...
  • 263
0 votes
0 answers
35 views

Polyextremal groups

A polynomial of a semigroup $X$ is a function $f:X\to X$ of the form $f(x)=a_0xa_1\cdots xa_n$, where $a_0,a_1,\dots,a_n$ some elements of the semigroup $X^1=X\cup\{1\}$, called the coefficients of ...
  • 35.5k
3 votes
1 answer
116 views

Length of representation of $GL_n(\mathbb{F}_q)$ in functions on Grassmannian

Let $G=GL_n(\mathbb{F}_q)$ be the (finite) group of all linear invertible transformations of the vector space $(\mathbb{F}_q)^n$ over the finite field $\mathbb{F}_q$. $G$ acts naturally on the ...
  • 19.4k
3 votes
0 answers
118 views

$2^2 \cdot U_6(2)$ and $2^2.2^{1+20}U_6(2)$ in $\mathbb{M}$

In the first diagram of this paper, there are conjugacy classes of subgroups of the Monster group which are labeled $2^2 \cdot U_6(2)$ and $2^2.2^{1+20}U_6(2)$, respectively. Can subgroups in the ...
0 votes
0 answers
50 views

Investigating the structure of a group algebra via the derived subgroup

It is well known that each element in the special linear group $\mathrm{SL}_n(\mathbb{H})$ over the real quaternion division ring with $n\geq1$ is a single multiplicative commutator. I am particularly ...
13 votes
2 answers
718 views

Which finite groups have low-degree essential cohomology?

Let $G$ be a finite group, $A$ some coefficients (e.g. $A = \mathbb{F}_2$ or $\mathbb{Z}$), and write $\mathrm{H}^\bullet_{\mathrm{gp}}(G; A)$ for the (ordinary) group cohomology of $G$ with ...

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