A group is an algebraic structure consisting of a set of elements together with an operation that satisfies four conditions: closure, associativity, identity and invertibility. Group theory studies groups.

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2
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1answer
534 views

a conjugate of a glide reflection by any isometry of the plane is again a glide reflection

Q : Prove that a conjugate of a glide reflection by any isometry of the plane is again a glide reflection, and show that the glide vectors have the same length. I know that: the conjugate ...
1
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1answer
21 views

Verifying generators of a cyclic group

Is there is a faster way to verify that a number is a generator of $Z_n^{*}$ other than taking the power of the number multiple times? For example, I want to know if 11 is a generator of ...
0
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0answers
13 views

Harmonic function on 3-dimensional lamplighter group

Can anyone give an example of a non-constant bounded harmonic function on the Lamplighter group $\mathbb{Z}_2 \wr \ \mathbb{Z}^3$? This function should exist, because the random walk on this group has ...
0
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2answers
23 views

How can we define that function?

I want to show that $M=\{\tau \in S_4\mid \tau (4)=4\}$ is isomorphic to $S_3$. To do that we have to consider a function $f(x)$ that gives the isomorphism of $M$ with $S_3$, i.e., we have to ...
11
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9answers
503 views

A good book for beginning Group theory

I am new to the field of Abstract Algebra and so far it's looking to me quite tough. So far I have encountered the following books in group theory - Contemporary abstract algebra by Joseph Gallian and ...
2
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1answer
23 views

Let $G$ be a $2$-group and suppose the centralizer of some element of order two has order at most four, then $G$ has maximal class

Let $G$ be a $2$-group of order $|G| \ge 4$ and $H \le G$ be a subgroup of order $2$, i.e. $|H| = 2$. Suppose we have $|C_G(H)| \le 4$. Then $G$ has maximal class. Do you know a proof of this fact? I ...
2
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1answer
35 views

Convincing normal subgroup proof?

I wrote the following proof on an exam, I was wondering if it makes sense. Question: let $H$ be a subgroup of $G$, written in multiplicitive notation. Prove that if the coset multiplication defined ...
3
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1answer
35 views

Are all countable torsion-free abelian groups without elements of infinite height free?

The height of an element $g$ in an abelian group $G$ is the largest $n\in \mathbb{N}$ such that there exist an element $h\in G$ such that $n*h=g$. If $g$ has no such largest integer than $g$ is of ...
8
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1answer
571 views

Semidirect product uniqueness argument for classifying groups of small order

I'm having trouble understanding the following method for determining the number of semidirect products between two groups in simple cases that arise when trying to classify certain groups of small ...
1
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1answer
38 views

Criterion for isomorphism of two groups given by generators and relations

When are two presentations of groups are isomorphic? In this post it is said: [...] find a set of generators of the first group that satisfies the relations of the second group [...] But I doubt ...
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0answers
47 views

Any good math books? [on hold]

I was wondering if there are any books about a bunch of random math theories, areas and topics. For example topics like group theory, randomness. klein bottles and other interesting things
1
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1answer
15 views

CG-homorphism proof. Stuck at the end!

I am trying to work on some questions back from my uni days, and one has gotten the better of me at the moment! Let $G$ be a finite group and $V, W$ finite-dimensional $\mathbb{C}G$-modules. Let ...
3
votes
2answers
54 views

Determine the number of subgroups of $\Bbb Z_p \times \Bbb Z_p$, where p is prime.

There are some answers online and we got one in our lecture. Unfortunately I have spent several hours trying to make sense of it and getting nowhere. I think it is mainly due to the fact of me being ...
0
votes
2answers
31 views

Subgroups of the multiplicative group $(\Bbb C^{\times},\cdot)$

I have $n\ge 2$ and have to show $U(n):=\{z\in\Bbb C\;:\; z^n=1\}$ is subgroup of the multiplicative group $(\Bbb C^{\times},\cdot)$. I can t understand the problem, help please.
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0answers
22 views

Space, symmetry and undecidability. [on hold]

So I went to a lecture a few months ago on symmetry. My question is at what point does symmetry or group theory become undecidable? How come I improve my understanding on trivial symmetry and ...
1
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0answers
40 views

Quick way to classify groups of small order

This is a past Qualifying exam on Algerbra : I'm curious if there is a quick way to solve Prob 1. To me, directly classifying groups takes too much time and I think I could not handle this in time ...
2
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1answer
33 views

$G$ be a group such that every maximal subgroup is of finite index and any two maximal subgroups are conjugate

Let $G$ be a group such that every maximal subgroup is of finite index and any two maximal subgroups are conjugate and any proper subgroup is contained in a maximal subgroup . Then is $G$ cyclic ? I ...
4
votes
2answers
25 views

Example of inverse semigroup with at least two idempotent elements

We say that the semigroup $S$ is inverse semigroup if for any $s\in S$ there is a unique $t\in S$ such that $sts=s,\ tst=t$. Suppose that $E(S)=\{e:\ e\in S,\ e^2=e\}$ and define $$s\sim ...
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0answers
14 views

Nonabelian $2$-groups whose derived length $l > 2$ [on hold]

Can we classify all nonabelian $2$-groups whose derived length is at least $3$?
0
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0answers
64 views

Is it correct? $1^n +2^n +…+(p-1)^n=-1 \pmod p$

$p$ a prime number, $n\in \mathbb{N} $ and $p-1\mid n$ then $1^n +2^n +...+(p-1)^n=-1 \pmod p$ I'm not sure if my proof is correct: Take the group $G=(\mathbb{Z}^{*}_{p},\cdot)$ with the ...
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0answers
37 views
+50

$G$ be a finite group and $f \in Aut (G)$ such that $f^3$ is identity and $f$ has unique fixed point , then any $p$-Sylow subgroup is normal?

Let $G$ be a finite group and $f \in Aut (G)$ such that $f^3$ is identity and $f(x)=x \implies x=e$ ; then is it true that for every prime $p$ dividing $|G|$ , there is exactly one $p$-Sylow subgroup ...
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0answers
14 views

Does irreducibility of a representation imply irreduciblity of all restricted representations?

Let $G$ be a group with a subgroup $H$. Then any representation of $G$ can be restricted to $H$. If the $G$ representation is irreducible then should the $H$ representation also be irreducible? If ...
4
votes
2answers
34 views

Isomorphism between $G$ and $\mathbb{Q}^{*}$

Let $\{G_{n}\}_{n\in \mathbb{N}}$ be a family of additive groups with $G_{1}=\mathbb{Z}_{2}$ and $G_{n}=\mathbb{Z}$ for $n\geq 2$ $$G=\bigoplus_{n\in \mathbb{N}}G_{n}$$ I want to prove that $G\cong ...
4
votes
2answers
157 views

When is a divisible group a power of the multiplicative group of an algebraically closed field?

It is known that for any algebraically closed field $\mathbb{F}$ its multiplicative group $\mathbb{F}^*$ is a divisible group, and consequently any power $\mathbb{F}^*\times\cdots\times \mathbb{F}^*.$ ...
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0answers
72 views
+50

How to count the closed left-hand turn paths of planar bicubic graphs?

When you draw a planar cubic bipartite graph $\Gamma$ and 3-color its edges you can use this as an orientation $\mathcal O$. Definition A left-hand turn path on $(\Gamma, \mathcal O)$ is a closed ...
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0answers
19 views

What's the motivation of the third condition of Nielsen-reduced?

The third condition of Nielsen-reduced is: $v_1v_2\ne$l and $v_2v_3\ne$l implies |$v_1v_2v_3$|> |$v_1$|-|$v_2$|+|$v_3$|.But I can't see why it's defined by this, how did he come up with such an idea?
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1answer
37 views

When does normal maximal subgroup have prime index?

Given finite group $G$, a normal maximal subgroup $H$, when is $[G:H]$ a prime? If $G$ is nilpotent, then the statement is true. But I am not sure about other $G$. Is there any counter-example ...
2
votes
1answer
21 views

perfect groups with non-trivial group homology over rational coefficients

A group $G$ is perfect if $G=[G,G]$. For perfect groups, we know that the first group homology $H_1(G, \mathbb{Z})=G/[G,G]=0$. A group $G$ is called acyclic if its group homology $H_i(G, ...
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1answer
21 views

Commutator subgroup $[H,K]$, $H, K$ subgroups of a group

How could I show that $[H,K]$ is a normal subgroup of $\langle H, K \rangle$? Also that if $H$ is generated by $X$ and $K$ is generated by $Y$, then $[H,K]=\langle g[x,y]g^{-1} | x \in X, y\in Y, g\in ...
1
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1answer
41 views

To prove that $G$ is the group the condition is not necessary $\forall a,b, c \in G(ba=ca\to b=c)$.?

$1.$ Let $G$ be a finite semigroup such that $\forall a,b, c \in G(ab=ac\to b=c)$. Then $G$ is Group. ? I know the following result : If $G$ be a finite semigroup such that $\forall a,b, c \in ...
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0answers
16 views

For the special class of $T$-subgroups in a certain quotient group the normal subgroups intersect certain subgroup

Let $G$ be a finite group and $U \le G$ be a subgroup of odd order which has index two in its normalizer and $U^g \ne U$ implies $U^g \cap U = 1$. Write $N_G(U) = TU$ with $T = \langle t \rangle$ for ...
204
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28answers
20k views

Nice examples of groups which are not obviously groups

I am searching for some groups, where it is not so obvious that they are groups. In the lectures script there are only examples like $\mathbb{Z}$ under addition and other things like that. I ...
2
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0answers
34 views

Classify all finite groups with property [on hold]

Classify all finite groups $G$ with the following property: for every $H\vartriangleleft G$ there exists $K<G$ such that $G/H$ is isomorphic to $K$. My poor abstract-algebraic imagination doesn't ...
1
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1answer
22 views

Order of a $\alpha \beta^{n/q}$ given the order of $\alpha$ and $\beta$

I'm asked to prove the following easy result: Let $G$ be a finite abelian group. Let $\alpha \in G$ of order $m$ and $\beta \in G$ of order $n$. Assume the $n\not\mid m$, and let $q=p^v$ for some ...
3
votes
3answers
62 views

A line avoiding an Algebraic group

Let $K$ be an algebraically closed field, and $G\subset (K,+)^3$ an algebraic subgroup (i.e. given as the zero sets of finitely many polynomial equations) of dimension 1. Is it clear that there is a ...
0
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0answers
35 views

A relation between a group and its subgroups [duplicate]

Let be $H$ a proper subgroup of finite group $G$. Who can we show that $G\not=\cup_{a \in G}aHa^{-1}$?
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1answer
50 views

how to find coefficient c1, c2, c3, c4 of a polynomials of degree 4 from resolvent

if not starting from standard resolvent of each degree and use (y-x1...)(y-x2...)(y-x3...) and group theory how to find corresponding c1, c2, c3, c4 of polynomial x^4+c4*x^3+c3*x^2+c2*x+c1 which c1, ...
0
votes
0answers
26 views

Sylow counting to show group is isomorphic to semidirect product

Let G be a group of order $|G|=pq^m$ where $p, q$ are primes with $q^m<p$. Use a Sylow counting argument to show that $G\cong C_p \rtimes_h Q$ where $Q$ is a group with $|Q|=q^m$ and ...
-3
votes
1answer
21 views

How to find identity element of a set (under modular) operation?

Question 1) Can the set of $\{0, 1, 2, 3\}$ under the operation of modulo-$4$ addition and multiplication form a group as well as a field ? If yes then how and if not then why ? Question 2) How to ...
3
votes
1answer
85 views

Trying to show $|ab|$ divides lcm$(|a|,|b|)$

I'm trying to solve this Putnam problem. The problem is "show that for a finite group with $n$ elements of order $p$, where $p$ is prime, either $n=0$ or $p\: \vert\: n+1$." I'm trying to do this by ...
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2answers
24 views

Order of group $G = \{A\in M_2(\mathbb{Z}_p): \mathrm{det}A= \pm 1 \}$

Also, $p>2$ is a prime number. Firstly, it's obvious that $G \leq GL_2(\mathbb{Z}_p)$, and we know that $|GL_2(\mathbb{Z}_p)|=p(p^2-1)(p-1)$. Next, we define the homomorphic map ...
2
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0answers
27 views

Using a Sylow Counting Argument

Let G be a group of order $$|G|=pq^m$$ where $p$ and $q$ are primes and $q^m<p$. Show that $$G\cong C_p \rtimes_h Q$$ where $Q$ is a group with $|Q|=q^m$ and $h:Q\rightarrow Aut(C_p)$ is a ...
1
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1answer
13 views

Abelianization of the absolute group and maximal abelian extension

Let $K$ be any field, $\overline K$ is the separable closure of $K$ and $K^{ab}$ is the maximal abelian extension of $K$. I want to prove the following relation $$G(\overline ...
2
votes
1answer
41 views

Galois group isomorphic to $\mathbb Z$

Does exist an example of a Galois extension $L/K$ such that $\text{Gal}(L/K)\cong \mathbb Z$? Thank you.
1
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1answer
36 views

How can I prove that G is abelian?

$N$ is a finite normal subgroup of order $n$ of $G$, and $|\operatorname{Inn}(G)|=m$ , $(n,m)=1$ and $[G:N]=p$ a a prime number. How can I prove that G is abelian? Can I use that $G$ is abelian iff ...
0
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1answer
33 views

Is/when is this property about the totative subset of $(\mathbb{Z}_n , +_n)$ true?

Is it possible to show (or when is it true); that for the group $\mathbb{Z}^+_n :=(\mathbb{Z}_n , +_n) $, there exists an $a \in \mathbb{Z}^+_n$ for each $z \in \mathbb{Z}^+_n$, where both $z+_n a$ ...
2
votes
1answer
22 views

Group of exponent $2$.

When I have a group $G$ of exponent $2$ and I know that all elements in $G-\{e\}$ are conjugated, is it right that $G$ is of order $2$? My try: For $g,h \in G - \{e\}$ the conjugation assumption ...
1
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1answer
35 views

Two non-isomorphic groups which are epimorphic images of each other

Let $G$ and $H$ be two groups, I am looking for an example such that $G$ is an epimorphic image of $H$ and $H$ is an epimorphic image of $G$ (i.e. they are both quotients of the other group), but they ...
1
vote
2answers
45 views

Group of finite order where every element has infinite order

An often given example of a group of infinite order where every element has infinite order is the group $\dfrac{\mathbb{(Q, +)}}{(\mathbb{Z, +})}$. But I don't see why every element necessarily has ...
2
votes
1answer
185 views

If a group mod its commutator is cyclic, then the group is abelian

If $G/G'$ is cyclic then G is abelian. Let G be a group and suppose $G/G'$ is cyclic.Then for all g in G we have $ g ∈ gG' = x^k G'$ for some integer k. In particular, $g=x^kz$ for some integer k and ...