Questions tagged [finite-groups]

Questions on group theory which concern finite groups.

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A problem with pointwise stabilizer subgroups of fixed-point subspaces II

Definitions: Let $W$ be a representation of a group $G$, $K$ a subgroup of $G$, and $X$ a subspace of $W$. Let the fixed-point subspace $W^{K}:=\{w \in W \ \vert \ kw=w \ , \forall k \in K \}$. Let ...
Sebastien Palcoux's user avatar
3 votes
2 answers
596 views

Kernel of modular representation of a finite group

Let $G$ be a finite group; let $F$ be a field of characteristic $p > 0$. If I have an irreducible modular representation $\rho: G \to GL_n(F)$, does $\ker \rho$ contain all the normal $p$-...
Dr Shello's user avatar
  • 1,160
2 votes
1 answer
279 views

Decomposition into irreducible of a representation of the wreath product $S_d \wr S_m$ (2)

This is a question following Decomposition into irreducible of a representation of the wreath product $S_d\wr S_n$ Let $F$ be the trivial and $S$ be the standard representations of $S_d$ (of ...
MarcO's user avatar
  • 553
2 votes
4 answers
539 views

relatively free groups in $Var(S_3)$

Suppose $S_3$ is the symmetric group of order 6. Which elements of the variety $Var(S_3)$ are relatively free? This question is related to my previous question Relatively free algebras in a variety ...
Sh.M1972's user avatar
  • 2,183
2 votes
0 answers
837 views

How to find quotients of infinite triangle groups or von Dyck groups?

I need the following information about the quotients of infinite triangle (or von Dyck) groups. (1) Let $G(l,m,n)$ defined as $S^l$ =$T^m$ = $(ST)^n$ = $E$ is the hyperbolic ($1/l+1/m+1/n<1$) ...
Ketan Patel's user avatar
2 votes
1 answer
294 views

An upper bound for the maximal subgroups at fixed index?

Let us call a subgroup an injective homomorphism between groups. I warn the reader that a subgroup designates here an inclusion $(H \subset G)$, not $H$ alone. A subgroup $H \subset G$ is ...
Sebastien Palcoux's user avatar
1 vote
0 answers
223 views

Does $[H_i , G_i]$ distributive imply $[H_1 \times H_2, G_1 \times G_2]$ modular?

Let $L(G)$ be the subgroup lattice of $G$ and $[H, G]$ an interval in $L(G)$. A lattice $(L, \wedge, \vee)$ is distributive if $a∨(b∧c) = (a∨b) ∧ (a∨c)$, $\forall a,b,c \in L $, and is modular if ...
Sebastien Palcoux's user avatar
0 votes
1 answer
453 views

Projective characters with corresponding factor set

The following is just a follow up to my previous question. I have a finite group $H$ with 14 ordinary characters. The Schur multiplier $M(H)\cong 2^2$. Hence the group $H$ will have 3 sets of ...
A.L. Prins's user avatar
97 votes
19 answers
36k views

Collecting proofs that finite multiplicative subgroups of fields are cyclic

I teach elementary number theory and discrete mathematics to students who come with no abstract algebra. I have found proving the key theorem that finite multiplicative subgroups of fields are cyclic ...
81 votes
3 answers
5k views

How do I verify the Coq proof of Feit-Thompson?

I probably don't have the appropriate background to even ask this question. I know next to nothing about formal or computer-aided proof, and very little even about group theory. And this question is ...
Nate Eldredge's user avatar
68 votes
20 answers
18k views

Fun applications of representations of finite groups

Are there some fun applications of the theory of representations of finite groups? I would like to have some examples that could be explained to a student who knows what is a finite group but does not ...
61 votes
1 answer
2k views

Are there $n$ groups of order $n$ for some $n>1$?

Given a positive integer $n$, let $N(n)$ denote the number of groups of order $n$, up to isomorphism. Question: Does $N(n)=n$ hold for some $n>1$? I checked the OEIS-sequence https://oeis.org/...
Peter's user avatar
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55 votes
2 answers
6k views

How do you *state* the Classification of finite simple groups?

From the point of view of formal math, what would constitute an appropriate statement of the classification of finite simple groups? As I understand it, the classification enumerates 18 infinite ...
Mario Carneiro's user avatar
54 votes
5 answers
10k views

Why are the sporadic simple groups HUGE?

I'm merely a grad student right now, but I don't think an exploration of the sporadic groups is standard fare for graduate algebra, so I'd like to ask the experts on MO. I did a little reading on them ...
REDace0's user avatar
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53 votes
5 answers
5k views

How much of the ATLAS of finite groups is independently checked and/or computer verified?

In a recent talk Finite groups, yesterday and today Serre made some comments about proofs that rely on the classification of finite simple groups (CFSG) and on the ATLAS of Finite Groups. Namely, he ...
David Roberts's user avatar
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51 votes
1 answer
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What is Atiyah's topological formulation of the odd order theorem?

Here is a quote from an article by Daniel Gorenstein on the history of the classification of finite simple groups (available here). During that year in Harvard, Thompson began his monumental ...
spin's user avatar
  • 2,781
49 votes
3 answers
2k views

Is each squared finite group trivial?

A semigroup $S$ is defined to be squared if there exists a subset $A\subseteq S$ such that the function $A\times A\to S$, $(x,y)\mapsto xy$, is bijective. Problem: Is each squared finite group ...
Taras Banakh's user avatar
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47 votes
1 answer
3k views

Which small finite simple groups are not yet known to be Galois groups over Q?

The subject line pretty much says it all. To expand just a little bit: 1) What is the smallest simple group that is not yet known to occur as a Galois group over $\mathbb{Q}$? (Variants: not known ...
Pete L. Clark's user avatar
47 votes
1 answer
3k views

Do all exact $1 \to A \to A \times B \to B \to 1$ split for finite groups?

Let $A$, $B$ be finite groups. Is it true that all short exact sequences $1 \rightarrow A \rightarrow A \times B \rightarrow B \rightarrow 1$ split on the right? In other words, do there exist ...
Dan Glasscock's user avatar
40 votes
1 answer
2k views

Known and fixed gaps in the proof of the CFSG

As the "second-generation" proof of the Classification of Finite Simple Groups is being written up in the volumes by Gorenstein, Lyons, Aschbacher, Smith, Solomon, and others (see e.g. this ...
Carl-Fredrik Nyberg Brodda's user avatar
39 votes
2 answers
4k views

Why are Schur multipliers of finite simple groups so small?

Given a finite simple group $G$, we can consider the quasisimple extensions $\tilde G$ of $G$, that is to say central extensions which remain perfect. Some basic group cohomology (based on the ...
Terry Tao's user avatar
  • 108k
38 votes
2 answers
4k views

Why does the monster group exist?

Recently, I was watching an interview that John Conway did with Numberphile. By the end of the video, Brady Haran asked John: If you were to come back a hundred years after your death, what problem ...
Leibniz's Alien's user avatar
37 votes
4 answers
6k views

For which $n$ is there only one group of order $n$?

Let $f(n)$ denote the number of (isomorphism classes of) groups of order $n$. A couple easy facts: If $n$ is not squarefree, then there are multiple abelian groups of order $n$. If $n \geq 4$ is even,...
Daniel Hast's user avatar
  • 1,806
37 votes
2 answers
2k views

A group-theoretic perspective on Frankl's union closed problem

Here is a group theoretic phrasing of a special case of the union closed conjecture: Question: Given a finite group $G$, is there an element of prime power order which is contained in at most half ...
Gjergji Zaimi's user avatar
35 votes
7 answers
4k views

Applications of Frobenius theorem and conjecture

A theorem of Frobenius states that if $n$ divides the order of a finite group $G$, then the number of solutions to $x^n = 1$ in $G$ is a multiple of $n$. Frobenius conjectured that if the number of ...
Mikko Korhonen's user avatar
32 votes
11 answers
13k views

Lecture notes on representations of finite groups

Next term I am supposed to teach a course on representation of finite groups. This is a third year course for undegrads. I was thinking to use the book of Grodon James and Martin Liebeck "...
31 votes
1 answer
2k views

Navigating $\mathbb{Z}/p\mathbb{Z}$

$\newcommand{\Z}{\mathbb{Z}}$Let's consider a silly-looking question first. Consider $\Z/p\Z$. Say I am allowed the two operations $x\mapsto x+1$ and $x\mapsto 2x$. Then, starting from $0$, I can ...
H A Helfgott's user avatar
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31 votes
5 answers
5k views

Generating a finite group from elements in each conjugacy class

Is there a finite group such that, if you pick one element from each conjugacy class, these don't necessarily generate the entire group?
Jamie Vicary's user avatar
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30 votes
1 answer
588 views

Guess that group via product queries

Suppose someone (person B) knows a finite group $G$ of order $n$. You (person A) know only the order $n$, and that $1$ is the name of the identity element. The group elements are named $1,2,\ldots,n$ ...
Joseph O'Rourke's user avatar
28 votes
0 answers
657 views

Mathieu group $M_{23}$ as an algebraic group via additive polynomials

An elegant description of the Mathieu group $M_{23}$ is the following: Let $C$ be the multiplicative subgroup of order $23$ in the field $F=\mathbb F_{2^{11}}$ with $2^{11}$ elements. Then $M_{23}$ is ...
Peter Mueller's user avatar
28 votes
3 answers
2k views

Being a subgroup: proof by character theory

Let me first cite a theorem due to Frobenius: Let $G$ be a finite group, with $H$ a proper subgroup ($H\ne (1)$ and $G$). Suppose that for every $g\not\in H$, we have $H\cap gHg^{-1}=(1)$. Then $...
Denis Serre's user avatar
  • 51.5k
28 votes
6 answers
1k views

Are there always more conjugacy classes in the kernel of a morphism to $Z_2$ than not?

Let $G$ be a finite group and let $\phi:G\to Z_2$ be a homomorphism to the group with two elements. Is it always the case that there are more conjugacy classes in the kernel of $\phi$ than conjugacy ...
Clark Lyons's user avatar
27 votes
2 answers
2k views

Monstrous Moonshine for Thompson group $Th$?

I. As a background, in Traces of Singular Moduli (p.2), Zagier defines the modular form of weight 3/2, $$g(\tau) = \frac{\eta^2(\tau)}{\eta(2\tau)}\frac{E_4(4\tau)}{\eta^6(4\tau)}=\vartheta_4(\tau)\, ...
Tito Piezas III's user avatar
27 votes
1 answer
2k views

Has anyone catalogued the "first generation" proof of the classification of finite simple groups?

It has been estimated that the original proof of the CFSG spans around 15,000 journal pages written by hundreds of authors over most of the 20th century. The GLS project attempted to simplify this ...
Mario Carneiro's user avatar
26 votes
3 answers
3k views

What is this subgroup of $\mathfrak S_{12}$?

On some occasion I was gifted a calendar. It displays a math quizz every day of the year. Not really exciting in general, but at least one of them let me raise a group-theoretic question. The quizz: ...
Denis Serre's user avatar
  • 51.5k
26 votes
3 answers
2k views

Highly transitive groups (without assuming the classification of finite simple groups)

What is known about the classification of n-transitive group actions for n large without using the classification of finite simple groups? With the classification of finite simple groups a complete ...
Noah Snyder's user avatar
  • 27.8k
25 votes
2 answers
4k views

In what sense is the classification of all finite groups "impossible"?

I think there is a general belief that the classification of all finite groups is "impossible". I would like to know if this claim can be made more precise in any way. For instance, if there is a ...
Keivan Karai's user avatar
  • 6,064
25 votes
0 answers
960 views

Is every $p$-group the $\mathbb{F}_p$-points of a unipotent group

Let $\Gamma$ be a finite group of order $p^n$. Is there necessarily a unipotent algebraic group $G$ of dimension $n$, defined over $\mathbb{F}_p$, with $\Gamma \cong G(\mathbb{F}_p)$? I have no real ...
David E Speyer's user avatar
24 votes
2 answers
684 views

What's the maximum probability of associativity for triples in a nonassociative loop?

In a finite nonabelian group, the probability that two randomly chosen elements commute cannot exceed 5/8. One easy proof also makes it easy to find the smallest groups that attain this bound, namely ...
John Baez's user avatar
  • 21.3k
23 votes
1 answer
1k views

Geodesics in finite groups

It seems that I can generalize a result from compact, connected Lie groups to finite groups, but in order to do so, I need to have some kind of geodesics on finite groups. Below is a proposition for ...
Joonas Ilmavirta's user avatar
23 votes
5 answers
5k views

Has any attempt been made to classify finite groupoids?

I recently stumbled upon the Mathieu groupoid and I found them fascinating. It appears as a subset of $S_{13}$ which is not closed under multiplication, but it turns out to be a groupoid with 13 ...
temp's user avatar
  • 1,990
23 votes
2 answers
2k views

Realizing groups as commutator subgroups

What are the groups $X$ for which there exists a group $G$ such that $G' \cong X$? My considerations: $\bullet$ If $X$ is perfect we are happy with $G=X$. $\bullet$ If $X$ is abelian then $G := X \...
Martino Garonzi's user avatar
22 votes
6 answers
2k views

What is the standard 2-generating set of the symmetric group good for?

I apologize for this question which is obviously not research-level. I've been teaching to master students the standard generating sets of the symmetric and alternating groups and I wasn't able to ...
Matthieu Romagny's user avatar
22 votes
1 answer
2k views

What finite simple groups we can obtain using octonions?

Rearranged on 2017-05-31 What I am missing is a uniform definition of finite simple groups. Especially sporadic groups are difficult to define. Many of the finite groups are defined using machinery ...
user avatar
21 votes
1 answer
1k views

McKay conjecture for finite groups in the simplest case G=GL(2,F_p) ( puzzle: Borel knows about cuspidals)

The McKay conjecture and related (Alperin, Issacs-Navarro) are one of the "main problems in the representation theory of finite groups" (G.Navarro pdf). Statement of the McKay conjecture is quite ...
Alexander Chervov's user avatar
21 votes
2 answers
1k views

A new combinatorial property for the character table of a finite group?

Let $G$ be a finite group and $\Lambda = (\lambda_{i,j})$ its character table with $\lambda_{i,1}$ the degree of the ith character. Consider the following combinatorial property of $\Lambda$: for ...
Sebastien Palcoux's user avatar
20 votes
4 answers
2k views

Cayley graph of $A_5$ with generators $(1,2,3,4,5),(1,4,3,2,5)$

The Cayley graph of $A_5$ with two generators of order 5 seems rather complicated. What is its graph genus (orientable or non-orientable)? The best I could get by trial and error is an embedding ...
Bjørn Kjos-Hanssen's user avatar
20 votes
3 answers
4k views

What is the geometric shape of the Monster sporadic group?

Conway made the comment that the Monster group represents the symmetries of a shape in 196,883 dimensions, something like a "star you hang on a Christmas tree." My question is, What do we know (or ...
JamesEadon's user avatar
19 votes
4 answers
1k views

The number of commuting m-tuples is divisible by order of group: Improvements?

The number of commuting pairs of elements in finite group G is equal to the product $k(G)*|G|$ (see MO271757 ) where $k(G)$ is the number of conjugacy classes. Thus it is is divisible by $|G|$ (the ...
Alexander Chervov's user avatar
19 votes
4 answers
3k views

determinant of the table of characters

I am certain that the answer to this question exists somewhere. It might be a classical exercise. Let $G$ be a finite group. Its table of characters is a square matrix, whose rows are indexed by the ...
Denis Serre's user avatar
  • 51.5k

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