Use with the (group-theory) tag. The tag "finite-groups" refers to questions asked in the field of Group Theory which, in particular, focus on the groups of finite order.

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7
votes
1answer
34 views

order of an elliptic curve

I have found that the curve given by $x^3+x+1=y^2$ over $\mathbb{F_5}$ has 9 points. Now I am supposed to find the number of points of the same curve on $\mathbb{F}_{125}$. Using Hasse and the fact ...
3
votes
1answer
238 views

normal p-subgroups of a finite group and chief factor

Let $G$ be a finite solvable group. Let $K/H$ be a chief factor of $G$ that is not of prime order, where $K$ is a $p$-subgroup of $G$ for some prime $p$ divides the order of $G$. Let $S$ be a proper ...
1
vote
2answers
52 views

Why is $1 + 1 = 0$ in $\{0, 1\}$ (binary field) and not 1 or 2?

Stuck on the simplest case in my foray into fields... I know there is a really similar question out there, but I can't find any contradiction with the field axioms if 1 + 1 = 1 instead of 0. Can ...
1
vote
2answers
39 views

$(a_1 a_2 \cdots a_n)^2=e$ in a finite abelian group

Let $G$ be a finite abelian group. Prove that $(a_1 a_2 \cdots a_n)^2=e$. My proof: $$\forall a \in \{a_1,a_2,\cdots,a_n\} \exists ! a^{-1} \in \{a_1,a_2,\cdots,a_n\}:a^{-1}a=aa^{-1}=e$$ hence, ...
3
votes
3answers
58 views

Group of even order must contain $a:a=a^{-1}$ $ (a\not = e)$ [duplicate]

Let $G$ be a finite group. If the order of $G$ is even, prove that there is at least one element $a$ in $G$ such that $a\not= e$ and $a=a^{-1}$. Here's my idea: Suppose $\{x_1,\cdots,x_n\}$ is ...
5
votes
2answers
64 views

If $p\mid|G|$ then how many elements of order $p$ are there in $G$?

Let $G$ be a finite group and $p$ be a prime such that $p\mid|G|$ , then obviously $G$ has an element of order $p$ (by Cauchy's theorem) ; I would like to know exactly how many elements of order $p$ ...
1
vote
6answers
61 views

Generators of the Symmetric group $S_3$

I am trying to find the generator(s) of the Symmetric Group $S_3$ and I have attempted this via brute force by listing the permutations of $S_3$ and composing and repeating them but I have not found ...
2
votes
1answer
177 views

proof that finite group of rotations of plane is cyclic

How can we prove that any finite group of orientation-preserving isometries of the Euclidean plane is cyclic, using the following hint? 'You may assume that given any non-empty finite set E in the ...
0
votes
1answer
13 views

The comultiplication on $\mathbb{C} Q $ for a matrix basis?

Let $G$ be a finite group and let $\mathcal{A} = \mathbb{C} G$ be the group Hopf algebra. The comultiplication on $\mathcal{A}$ is well-known to be given by $\Delta(g) = g \otimes g$. For $G=Q$ the ...
2
votes
1answer
64 views

The comultiplication on $\mathbb{C} S_3$ for a matrix basis?

Let $G$ be a finite group and let $\mathcal{A} = \mathbb{C} G$ be the group Hopf algebra. The comultiplication on $\mathcal{A}$ is well-known to be given by $\Delta(g) = g \otimes g$. For $G=S_3$, ...
2
votes
1answer
64 views

Proving Finiteness of Group from Presentation

Given the group $G = \langle a, b, c : a^2 = b^3 = c^5 = abc\rangle$, I want to show that $H = G / \langle abc\rangle$ is a finite group. I tried to find a canonical form for elements of $H$. That ...
0
votes
1answer
78 views

How does $A_n$ look in Aut$(X)$?

Let me phrase my question precisely: Let $X=\{1,2,3,...,n\}$, $ \ S_n=\mbox{Sym}\{1,2,3,...,n\}$ be symmetric group on $n-$letters. Let $\mbox{Aut}(X)$ denote the automorphism group of $X$. We ...
4
votes
1answer
69 views

Computation of the cokernel of the map $f: \mathbb Z \rightarrow \mathbb Z_{(2)} \oplus \mathbb Z $ defined by $f(1)=(1,2)$

$f: \mathbb Z \rightarrow \mathbb Z_{(2)} \oplus \mathbb Z $ (the sum is direct) $f(1)=(1,2)$ so the image is $\mathbb Z*(e_{1}+2e_{2})$ however computing the cokernel of this map really puzzles me ...
8
votes
3answers
379 views

Finding an explicit isomorphism between $\mathbb{Z}^{\times}_n$ and $\mathbb{Z}^{\times}_{2n}$

For an odd integer $n$, find an explicit isomorphism between $\mathbb{Z}^{\times}_n$ and $\mathbb{Z}^{\times}_{2n}$. How do I do this? I don't really know where to start. I can easily find bijections ...
4
votes
1answer
490 views

Exercises in Group Cohomology

I'm interested in finding a textbook to learn group cohomology, a book that contains a lot of examples and also a lot of good exercises to test my understanding. I would appreciate some feedback. ...
7
votes
1answer
198 views

Maximal Subgroups and order of a group

I encountered the following exercise in Isaacs' Algebra: "Suppose a group $G$ has only one maximal subgroup. Prove that the order of $G$ must be a power of a prime". I think I've proven this for ...
4
votes
0answers
48 views

Variational formulations in group theory?

I apologise if this is a naïve question. Are there any known / widely applicable / important variational formulations in (finite) group theory? That is, a relationship of the form $$\alpha(G) = ...
1
vote
1answer
69 views

How do we know the classification of finite simple groups is finished?

I don't have much experience in group theory beyond knowing what a finite simple group is, as well as the other basic parts of the topic, so far as I can tell, so if the actual answer is too complex ...
4
votes
1answer
43 views

How to determine the isomorphism types of given groups with generators and relations

I was classifying the all groups of order 30 and I got the following groups $\langle a,b \mid b^{-1}ab=a^4, a^{15}=b^{2}=1\rangle$ and $\langle a,b \mid b^{-1}ab=a^{11}, a^{15}=b^{2}=1\rangle$. How ...
1
vote
0answers
52 views

Let $G$ be a group and $K$ be subgroup of order $p^a$. I am trying to show that $|\{H \leq G : K \subset H, |H|=p^{a+b}\}| \equiv 1 \pmod{p}$

Let $G$ be a group and $K$ be subgroup of order $p^a$. I am trying to show that $$|\{H \leq G : K \subset H, |H|=p^{a+b}\}| \equiv 1 \pmod{p}$$ for each b fixed & for all $a+b$ such that ...
2
votes
2answers
55 views

A Gap code for the alternating group $A_4$

I need a GAP code for checking the following question: Is it true that for every subset $A$ of the alternating group $A_4$ with $4$ elements there exists a subset $B$ of order $3$ such that ...
2
votes
1answer
411 views

Subgroups and quotient groups of finite abelian $p$-groups

Motivation The fundamental theorem of finite abelian groups gives us a concise description of the isomorphism types of finite abelian $p$-groups $G$ (in the following, $p$ is a fixed prime). The ...
1
vote
2answers
23 views

Does the following binary operation form a group on a set with 3 elements? (multiple identities?)

Let S = {a, b, c}. *| a b c ----------- a| a b c b| b a a c| c a a This seems to have all the desired characteristics of a group, however, both b and c ...
1
vote
0answers
23 views

How to reconstruct geometric object that a Frobenius group acts on

A Frobenius group has equivalent definitions: a transitive permutation group on a finite set such that no non-trivial element fixes more than one point and some non-trivial element fixes a point. ...
0
votes
0answers
50 views

Next generation of GAP Data Library “Transitive Permutation Groups”? [closed]

The current GAP Data Library "Transitive Permutation Groups" contains the transitive permutation groups of degree up to $30$. Question: The next generation of this Data Library will be with degree ...
0
votes
0answers
8 views

A question about relation between a finite simple group and a linear algebraic group

let $G$ be a linear algebraic group of type $\mathcal{X}$ over an algebraic closed field of characteristic $p$, $K$. suppose $F:G\rightarrow G$ is a Frobenius map and $\mathcal{G}=G^F$ is a finite ...
2
votes
1answer
53 views

$GL_2(R)$ and $\mathbb{A}_4$

So, i have to prove that $\mathbb{A}_4$ is not isomorphic to a subgroup $G$ of $GL_2(R)$. Here is a "hint" (are previuos part of the same problem that i was thinking, so i supose should help): If ...
2
votes
3answers
62 views

Motivation and intuition of double cosets

In Dummit and Foote, there was a problem on double cosets. Let $H$ and $K$ be subgroups of $G$ and $x\in G$, $HxK$ is the double coset of $x$. I can prove the double cosets partitions $G$. However, I ...
0
votes
2answers
30 views

Show that $\mathbb{F}_p^\times \simeq \text{Aut}(\mathbb{F}_p^+) $ holds

I want to show, that $\mathbb{F}_p^\times \simeq \text{Aut}(\mathbb{F}_p^+)$ holds with $p$ prime. ($\mathbb{F}_p^+$ is the additive group, $\mathbb{F}_p^\times$ multiplicative group) As hint we ...
1
vote
1answer
32 views

Proof of the theorem that relates the size of the conjugacy class to the order of the centralizer subgroup

Could you please explain the step in this theorem and proof that I do not follow: If $G$ is a finite group, let $C_{G}(x)$ be the centralizer of $x$ in $G$, that is $C_{G}(x)=\{g \in G: xg=gx\}$ ...
4
votes
2answers
103 views

Normalizer and centralizer of abelian subgroups of a group are equal [closed]

I have a question: Let $G$ be a finite group. If for each abelian subgroup $H$ of $G$ the centralizer and the normalizer of $H$ are equal, that is, $C_G(H)=N_G(H)$, prove that $G$ is abelian ...
1
vote
1answer
25 views

Frobenius dihedral groups

How can we show by a direct group-theoretic proof that the dihedral group $D_{2n}$ is a Frobenius group iff $n$ is an odd number?
0
votes
2answers
64 views

Use Sylow's theorem to show that $G = HN_G(P)$

I am stumped on this question. Does anyone have some helpful hints or a solution to this question? Thanks! Let $G$ be a group of finite order. Let $H$ be a normal subgroup of $G.$ Let $P$ be a ...
5
votes
2answers
272 views

Check whether two subgroup of $GL(n,\mathbb Z)$ are conjugate

Suppose I have two finite subgroups of $GL(n,\mathbb Z)$. Is there an algorithm to find out whether these two belong to the same conjugacy class in $GL(n,\mathbb Z)$? I tried by using the Jordan ...
10
votes
3answers
88 views

Group of order $224$

Any one can give a hint to prove this? Every group of order $224$ have a subgroup of order $28$.
5
votes
3answers
239 views

Is there an elegant way to determine which subgroups of $S_3$ are normal?

I have a homework problem which reads List all subgroups of $S_3$ and determine which subgroups are normal and which are not normal. I understand the definitions of subgroup and normal subgroup, ...
3
votes
1answer
100 views

Subgroup relations in $GL(3,\mathbb Z)$

There are 73 conjugacy classes of finite subgroups in $\operatorname{GL}(3,\mathbb{Z})$. If you take 73 representatives, you will find group-subgroup relations between them. There must exist an ...
4
votes
2answers
366 views

Geometrical meaning of automorphisms of cyclic groups

I'm looking for a geometrical interpretation of the action of automorphisms of cyclic groups. I'll take one particular example to make it clear : I'm taking the cyclic group $\mathbb{Z}_{12}$, which ...
3
votes
3answers
173 views

The product of all the elements of a finite abelian group

I'm trying to prove the following statements. Let $G$ be a finite abelian group $G = \{a_{1}, a_{2}, ..., a_{n}\}$. If there is no element $x \neq e$ in $G$ such that $x = x^{-1}$, then $a_{1}a_{2} ...
5
votes
1answer
546 views

Cyclic groups whose every non-identity member is a generator

Let $G$ be a cyclic group. There's a theorem which states that if $|G|$ is a prime, then every non-identity member of $G$ is a generator. What about a cyclic group whose order is not prime: Is there ...
4
votes
0answers
131 views

a split exact sequence of finite groups

Suppose G has a cyclic normal subgroup $\langle a\rangle$ of order $m$ and prime power index $s$ such that $m$ and $s$ are relatively prime. Then the following exact sequence splits: $$1 ...
3
votes
1answer
96 views

A Group of Order $540$ is not simple

Why is a group of order $540$ not simple? The hints I have been given are not helpful. Here's what I have been told. Let $G$ be such a group. Then there are $36$ Sylow $5$-subgroups; let $H$ be ...
7
votes
0answers
329 views

Why is the Monster group the largest sporadic finite simple group?

I do know that there is a "long proof" (I have read that it is +1000pages) that the Monster group is the largest sporadic simple group. My question is: Why can we be sure that there is no other ...
3
votes
5answers
201 views

Cayley's Theorem question: examples of groups which aren't symmetric groups.

Basically, Cayley's Theorem says that every finite group, say $G$, is isomorphic to a subgroup of the group $S_G$ of all permutations of $G$. My question: why is there the word "subgroup of"? If we ...
-1
votes
1answer
133 views

Isomorphisms between symmetric, dihedral and cyclic groups

What examples are there of isomorphisms between the groups $S_n,\, D_n, \, \mathbb{Z}_n$? Thank you.
5
votes
1answer
164 views

Why do we care about simple groups rather than indecomposable groups?

A first thing that we do to analyze something is "divide and conqur." So it is natural to consider simple groups. In the same way, it appears to be natural to consider indecomposable groups. However, ...
7
votes
6answers
400 views

Why does $x$ vanish in the group $\langle x,y\mid x^3, y^3, yxyxy\rangle$?

This comes from Artin Second Edition, page 219. Artin defined $G = \langle x,y\mid x^3, y^3, yxyxy\rangle$, and uses the Todd-Coxeter Algorithm to show that the subgroup $H = \langle y\rangle$ has ...
4
votes
2answers
106 views

Subgroups of groups of order 36

Is there any group of order 36 with no subgroup of order 6? Is there any group of order $p^2q^2$ with no subgroup of order $pq$? Is there any group of order $p^{2m}q^{2m}$ with no subgroup of order ...
2
votes
1answer
49 views

Is there any efficient algorithm for finding subgroups of a given finite group?

I am implementing an algorithm which finds every subgroup of given group. Here's my algorithm. Let $G$ be a group of order $n$ with elements $g_1,\cdots,g_n$. Then I consider each $\langle ...
1
vote
4answers
92 views

If $G$ is a non-cyclic group of order $n^2$, then $G$ is isomorphic to $\mathbb{Z_n} \oplus \mathbb{Z_n}$

I've independently come up with a question (I know it's been asked before, but I can't find the question online) involving the external direct product, non-cyclic groups and isomorphisms. So, is the ...