The study of symmetry: groups, subgroups, homomorphisms, group actions.

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1answer
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Prove $Q_8$, the group generated by two complex matrices $A$ & $B$ (see below) is a nonabelian group of order 8.

Problem: Let $Q_8$ be the group (under ordinary matrix multiplication) generated by the complex matrices $A=\begin{pmatrix} 0 & 1 \\ -1 & 0 \end{pmatrix}$ and $B=\begin{pmatrix} 0 & i \\ i ...
2
votes
1answer
21 views

Please check my proof on: $\sim$ is an equivalence relation $\Leftrightarrow S<G$

Problem: Let $\emptyset\ne S\subset G$, where $G$ is a group, and define a relation on $G$ by $a\sim b\Leftrightarrow ab^{-1}\in S$. Show that $\sim$ is an equivalence relation if and only if $S$ is a ...
0
votes
1answer
16 views

Countably generated group has at most countably many finite index subgroups

I know that if $G$ is a finitely generated group, then $G$ has at most countably many finite index subgroups. Is this result still true if $G$ is countably generated?
2
votes
1answer
20 views

Why does $x^{m \cdot 2^i} \equiv -1$ with odd $m$ imply that $x$ has order $m \cdot 2^{i+1}$?

It is clear that $$x^{m \cdot 2^{i+1}} \equiv 1$$ for odd $m$ but is there a theorem or an obvious reason why $x$ cannot have order smaller than $m \cdot 2^{i+1}$? Context: I am trying to understand ...
2
votes
0answers
23 views

Almost pointwise inner automorphism of free products of groups.

Let $A,B$ be groups, let $G = A\ast B$ be their free product and let $\phi \in \text{Aut}(G)$ be a automorphism of $G$. We say that $\phi$ is pointwise inner if $\phi(g) \sim_G g$ (there is $w \in G$ ...
2
votes
1answer
26 views

Semidirect products as (amalgamated) free product

It is well known that $\mathbb{Z}\rtimes \mathbb{Z}_2$ is free product $\mathbb{Z}_2 \star \mathbb{Z}_2$. Are there more examples of these kind?
0
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0answers
58 views

Prove $H_1 \cap H_2 \le H_1 $ when $H_1, H_2 \le G$ and $H_1$, $H_2$ are finite.

Prove that $H_1 \cap H_2 \le H_1 $ when $H_1, H_2 \le G$ and $H_1$, $H_2$ are finite. What I've done Use the definition of subgroup: $G$ is a group and $H \subseteq G$. $H \le G \iff HH=H $ and ...
5
votes
2answers
237 views

Is there a group-theoretic proof of the Riemann rearrangement theorem?

The analytic proofs of the Riemann rearrangement theorem are easy to understand but they don't explain why commutativity breaks down when you go from finite sums to infinite sums of real numbers. I ...
0
votes
3answers
56 views

If $H$ and $K$ are subgroups of G then $H \times K$ is a subgroup of $G \times G$

I know that if $H$ and $K$ are subgroups of $G$ then $HK= \{ hk \mid h \in H , k \in K\}$ is not necessarily a subgroup of $G$, this requires that $HK = KH$. But it follows that if $H$ and $K$ are ...
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0answers
46 views

Give a examples of $p$-Sylow subgroups of $G=A_5\times S_3$ for $p=2,3,5$ and tell what group is isomorphic to each subgroup? [on hold]

Give a examples of $p$-Sylow subgroups of $G=A_5\times S_3$ for $p=2,3,5$ and tell what group is isomorphic to each subgroup? Does there exist a subgroup of $G$ of order $180$?
1
vote
1answer
45 views

Irreducible action of a group on a set

Let $p$ be a prime. I am solving a problem and I am told that I should use that the action of $\text{SL}_2(p)$ on $\mathbb{F}_p^2$ is irreducible. But I don't know what this means?
2
votes
1answer
36 views

Lower central series and the center of $S/S^k$

Let $S$ be a $p$-group and assume that $S$ has maximal class - so if we assume $\vert S \vert = p^n$ then the lower (and upper) central series has length $n-1$. I don't know much about lower central ...
2
votes
2answers
44 views

How to find $[A_n,A_n]$

Let $n \in \mathbb{N}$. How could I find $$ [A_n,A_n] \quad \cong \quad \langle ghg^{-1}h^{-1} \ : \ g,h \in A_n \rangle $$ My own thoughts I rememberder that any element in $A_n$ can be written ...
4
votes
2answers
72 views

Realizing $\mathbb{Z}/n \mathbb{Z} \oplus \mathbb{Z}/n\mathbb{Z}$ as an isometry group

Every finite group arises as the isometry group of a subspace of an euclidean space (Albertsona, Boutin, Realizing Finite Groups in Euclidean Space). What are natural examples of spaces realizing the ...
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0answers
22 views

Can we extend this presentation as an infinite vector?

The motivation to this question can be found in: How I can express $(x,y)∈G$ by using the $r$ independent points $P_1,P_2,\ldots,P_r$ My question is: The group $C(ℚ)$ is a finitely generated Abelian ...
3
votes
2answers
104 views

Is an arbitrary group generated by a traversal of the conjugacy classes?

Let $G$ be a group, and let $\mathcal C$ be the collection of conjugacy classes of $G$. Let $S$ be a traversal of $\mathcal C$ (that is $S$ contains exactly one element from each set in $\cal C$). ...
0
votes
1answer
35 views

what are the other 2 nontrivial elements of the automorphism group of $\Bbb Z/5\Bbb Z$?

It is known that the automorphism group of the units of $\Bbb Z/5\Bbb Z$ is isomorphic to the cyclic group of order $4$, so the automorphism group must also have $4$ elements. The two nontrivial ones ...
3
votes
2answers
379 views

Is isomorphism not always unique?

Given two isomorphic groups G and H, is it possible that two or more functions define their isomorphism? Also, is it possible that another group say, L is isomorphic to G but not to H?
7
votes
1answer
45 views

Solvable groups and orders of elements

I am trying to prove the following result. Let $G$ be a group containing elements $x$ and $y$ such that the orders of $x$, $y$, and $xy$ are pairwise relatively prime; prove that $G$ is not ...
3
votes
2answers
148 views

Cyclic Group Presentation [on hold]

Show that the the group with presentation $$\langle x, y\ \mid\ x^2=y^2x^2y,\ (xy^2)^2=yx^2, \ yx^{-1}y^2=x^7\rangle $$ is cyclic of order 24. This presentation was obtained using the Todd-Coxeter ...
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2answers
65 views

Free action by cyclic group.

Let $G$ be a group acting on a set $X$. If $g\in G$ has no fixed points, prove or disprove the cyclic group $\left \langle g \right \rangle$ acts freely on $X$. edit: Can also assume $g$ has finite ...
2
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1answer
27 views

Finite groups with a cyclic maximal subgroup.

In the book A Course in the Theory of Groups by Derek J.S. Robinson, Finite $p$-groups with a cyclic maximal subgroup are classified. Now I wish to know whether finite groups with a cyclic maximal ...
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vote
0answers
36 views

Can the derived subgroup be realized as an intersection of stabilizers?

For any group $G$, we have that $Z(G)=\bigcap_{x\in G}C_G(x)$ and that each $C_G(x)$ is the stabilizer of $x$ when $G$ acts on itself by conjugation. Is there a similar representation for $G'$? That ...
1
vote
1answer
43 views

Finitely generated, periodic group such that each conjugacy class is finite must be finite?

This is essentially a repeat of this question. However, the OP didn't seem to put in any work towards a solution and didn't provide context. Nevertheless, I'm still interested in the solution, and I ...
1
vote
1answer
46 views

Finitely generated group 3 [duplicate]

Let $G$ be a group generated by a finite subset $X$, such that Every element of $X$ has finite order; The number of $G$-conjugates of any element of $X$ is finite. Is it true that $G$ is ...
-2
votes
0answers
26 views

Number of normal subgroups of a non-abelian group of order 21 [on hold]

How many normal subgroups can a non-abelian group G of order 21 have other than the identity subgroup {e} and G? a)0 b)1 c)3 d)7
0
votes
0answers
17 views

Existence of Generalized Hadamard matrices

Does there exist a Generalized Hadamard matrix of order 20 over an Elementary abelian group of order 4 ,GH(20,EA(4))?
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1answer
46 views

A question about Tarski Monster Group

Let $\alpha$ be a cardinality. Is there a Tarski moster group with exacly $\alpha$ non-trivial proper subgroups‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌‌?
0
votes
1answer
40 views

Absolutely and Relatively free abelian groups.

I see that there are the notions of absolutely free abelian group and relativley free abelian group. Could you please explain the difference between the two notions. Thanks!!
3
votes
1answer
38 views

Free cyclic subgroups in a non-abelian group

Is there any non-abelian group $G$ such that for each $a\in G$ and any automorphism $g:\left<a\right>\to \left<a\right>$ the function $$f:G\to G$$ $$f(x) = \begin{cases} g(x) & \text{ ...
6
votes
0answers
56 views

I would like to show that all reflections in a finite reflection group $W :=\langle t_1, \ldots , t_n\rangle$ are of the form $wt_iw^{-1}.$

I would like to show that all reflections in a finite reflection group $W := \langle t_1, \ldots , t_n\rangle$ are of the form $wt_iw^{-1}$ for some $i$ and some $w \in W$ Clearly all such elements ...
-1
votes
0answers
45 views

Finitely generated groups 3 [on hold]

Let $G$ be a group generated by a finite subset $X$, such that Every element of $X$ has finite order; The number of $G$-conjugates of any element of $X$ is finite. Is it true that $G$ is ...
0
votes
1answer
44 views

Inconsistent definition of Sylow p-subgroup

Here is the definition of a Sylow $p$-subgroup from Wikipedia: For a prime number $p$, a Sylow $p$-subgroup (sometimes $p$-Sylow subgroup) of a group $G$ is a maximal $p$-subgroup of $G$, i.e., a ...
6
votes
1answer
80 views

About translating subsets of $\Bbb Z.$

This is a continuation of About translating subsets of R2. Is it possible to find a pair of sets $A,B\subseteq\Bbb Z$ such that A is a union of translated (only translations are allowed) copies of ...
2
votes
1answer
28 views

Constructing an indicator function from a braid group which represents 'all strings have returned to their initial position'.

TL;DR Is there a well-defined closed formula from the braid group $B_n$ to $\{-1,1\} \left(\text{ or }\{0,1\}\right)$ which represents whether all the strings have returned to their initial ...
0
votes
1answer
29 views

What are the elements of Z/10Z and of Z/2Z×Z/5Z, and identify which elements correspond under the map g from Z/10Z to Z/2Z × Z/5Z. [on hold]

What are the elements of Z/10Z and of Z/2Z×Z/5Z, and identify which elements correspond under the map g from Z/10Z to Z/2Z × Z/5Z. I know the elements of Z/10Z are {1,3,7,9}, is that the same for ...
1
vote
1answer
23 views

Prove that $K=k(\alpha)$

Prove: If K\k is a Galois extension and $\alpha \in K$ with $\sigma(\alpha)\neq \alpha$ for all $\sigma \in Gal(K,k)\backslash \lbrace id_K \rbrace$, than $K=k(\alpha)$.
5
votes
3answers
106 views

Finitely-generated group such that all (non-trivial) normal subgroups have finite index implies all (non-trivial) subgroups have finite index?

Let $G$ be a finitely generated group such that every non-trivial normal subgroup has finite index. Does it follow that every non-trivial subgroup of $G$ has finite index? This question arose as ...
0
votes
2answers
66 views

We have a map $g : \mathbb{Z}/24\mathbb{Z} → \mathbb{Z}/6\mathbb{Z} \times \mathbb{Z}/4\mathbb{Z} $. What is the kernel of $g$?

We have a map $g : \mathbb{Z}/24\mathbb{Z} → \mathbb{Z}/6\mathbb{Z} \times \mathbb{Z}/4\mathbb{Z}$ given by $g(x+24\mathbb{Z}) = (x + 6\mathbb{Z}, x + 4\mathbb{Z})$. What is the kernel of $g $? In ...
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vote
1answer
85 views

Trivial elements in $T(a,b,c)$

Consider the group $T(a,b,2)=<x,y|x^a, y^b, (xy)^2>$ and assume none of $a$ or $b$ is equal to $2$. How can one list all the trivial words (say up to length $11$ and apart from $(xy)^{2n})$) in ...
2
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0answers
65 views

Cokernel of injective endomorphisms of a finitely generated free abelian group

By $\text{GL}^+(n,\mathbb{Z})$ we mean the set of $n×n$ invertible matrices with positive determinant and entries from $\mathbb{Z}$. For given $A \in \text{GL}^+(n,\mathbb{Z})$ let ...
2
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0answers
64 views

Groups such that all elements of even order are in $G'$

We know that $G=A_4$ is a group such that all elements of even order in $G$ are in $G':=[G,G]= V_4$, the klein four group. Are there other examples or classes of groups where all elements of even ...
1
vote
1answer
26 views

If the quotient by the $i$th center is cyclic, does it follow that the original group is abelian?

Let $G$ be a group such that there exists an $i$ such that $G/Z^i(G)$ is cyclic. Does it follow that $G$ is abelian? This question is a generalization of the well known fact that if $G/Z(G)$ is ...
1
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1answer
40 views

Are these exactly the abelian groups (2)?

This is a continuation of Are these exactly the abelian groups? I would like to consider another condition on a group and see if it implies commutativity. The condition is $$(\forall A,B\subseteq ...
2
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1answer
40 views

$\mathbb{F}_2 \times \mathbb{F}_2$ is not subgroup separable

I read that $\mathbb{F}_2 \times \mathbb{F}_2$ is not subgroup separable (ie. for every finitely generated subgroup $H$ and $g \notin H$, there exists a finite index subgroup $K$ such that $H \subset ...
1
vote
1answer
60 views

Bound on the index of an abelian subgroup in discrete subgroup of the euclidean group?

$\DeclareMathOperator{\isom}{Isom}$A discrete subgroup of the group of isometries in euclidean space is almost abelian. By this I mean that for each $n$ there exists $m$ such that for any discrete ...
2
votes
1answer
55 views

Write cyclic groups of order $p^n$ in terms of simple groups

Some say that studying simple groups helps you understand the structure of non-simple groups. How can I write in terms of simple groups $\mathbb{Z}_{p^n}$? Eg. $\mathbb{Z}_9$
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2answers
78 views

Abelianization of the free product of two cyclic groups.

Suppose that $G=G_1*G_2$ where $G_1$ and $G_2$ are cyclic of orders $m$ and $n$ respectively. Show that $G/[G,G]$ has order $mn$. Can anyone help me?
2
votes
1answer
33 views

Commuting Elements in a Free Product of Cyclic Groups

In the free group with two generators $F_2\cong\mathbb Z *\mathbb Z$ ($*$ denotes the free product), if two elements $a$ and $b$ commute, then there exists an element $w\in F_2$ such that $\langle ...
1
vote
0answers
29 views

To derive commutativeness of any group from the normality of all its subgroups & some other conditions

If a group is abelian then it is known that every subgroup of the group is normal ; is the converse true i.e. if every subgroup of a group is normal , then is it true that the grroup is abelian ? If ...