Infinite Abelian subgroup of infinite non Abelian group example
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My thought is that we may take GL(2,F) as the group and this is obviously infinite and non abelian since matrix multiplication does not commute. Then I thought that if we make $langle grangle$, for some $g$ in $mathrm{GL}(2,F)$, which will be cyclic and hence Abelian, for instance:
$ g=
bigg[
begin{matrix}
1&0\0&2
end{matrix}
bigg]
$. Then $g^n$ will be in the form $ g^n=
bigg[
begin{matrix}
1&0\0&2^n
end{matrix}
bigg]
$. This is obviously infinite since $g^n=e Leftrightarrow n = 0$.
Would this example work? Much thanks in advance!
abstract-algebra group-theory
$endgroup$
add a comment |
$begingroup$
My thought is that we may take GL(2,F) as the group and this is obviously infinite and non abelian since matrix multiplication does not commute. Then I thought that if we make $langle grangle$, for some $g$ in $mathrm{GL}(2,F)$, which will be cyclic and hence Abelian, for instance:
$ g=
bigg[
begin{matrix}
1&0\0&2
end{matrix}
bigg]
$. Then $g^n$ will be in the form $ g^n=
bigg[
begin{matrix}
1&0\0&2^n
end{matrix}
bigg]
$. This is obviously infinite since $g^n=e Leftrightarrow n = 0$.
Would this example work? Much thanks in advance!
abstract-algebra group-theory
$endgroup$
3
$begingroup$
If $A$ is an infinite abelian group and $H$ is a finite, non-abelian group then $Atimes H$ works. [Also, you could take $F=mathbb{Z}$ in your example to get something easy to work with, but which isn't a field :-) ]
$endgroup$
– user1729
12 hours ago
add a comment |
$begingroup$
My thought is that we may take GL(2,F) as the group and this is obviously infinite and non abelian since matrix multiplication does not commute. Then I thought that if we make $langle grangle$, for some $g$ in $mathrm{GL}(2,F)$, which will be cyclic and hence Abelian, for instance:
$ g=
bigg[
begin{matrix}
1&0\0&2
end{matrix}
bigg]
$. Then $g^n$ will be in the form $ g^n=
bigg[
begin{matrix}
1&0\0&2^n
end{matrix}
bigg]
$. This is obviously infinite since $g^n=e Leftrightarrow n = 0$.
Would this example work? Much thanks in advance!
abstract-algebra group-theory
$endgroup$
My thought is that we may take GL(2,F) as the group and this is obviously infinite and non abelian since matrix multiplication does not commute. Then I thought that if we make $langle grangle$, for some $g$ in $mathrm{GL}(2,F)$, which will be cyclic and hence Abelian, for instance:
$ g=
bigg[
begin{matrix}
1&0\0&2
end{matrix}
bigg]
$. Then $g^n$ will be in the form $ g^n=
bigg[
begin{matrix}
1&0\0&2^n
end{matrix}
bigg]
$. This is obviously infinite since $g^n=e Leftrightarrow n = 0$.
Would this example work? Much thanks in advance!
abstract-algebra group-theory
abstract-algebra group-theory
edited 12 hours ago
user1729
17.6k64294
17.6k64294
asked 12 hours ago
JustWanderingJustWandering
562
562
3
$begingroup$
If $A$ is an infinite abelian group and $H$ is a finite, non-abelian group then $Atimes H$ works. [Also, you could take $F=mathbb{Z}$ in your example to get something easy to work with, but which isn't a field :-) ]
$endgroup$
– user1729
12 hours ago
add a comment |
3
$begingroup$
If $A$ is an infinite abelian group and $H$ is a finite, non-abelian group then $Atimes H$ works. [Also, you could take $F=mathbb{Z}$ in your example to get something easy to work with, but which isn't a field :-) ]
$endgroup$
– user1729
12 hours ago
3
3
$begingroup$
If $A$ is an infinite abelian group and $H$ is a finite, non-abelian group then $Atimes H$ works. [Also, you could take $F=mathbb{Z}$ in your example to get something easy to work with, but which isn't a field :-) ]
$endgroup$
– user1729
12 hours ago
$begingroup$
If $A$ is an infinite abelian group and $H$ is a finite, non-abelian group then $Atimes H$ works. [Also, you could take $F=mathbb{Z}$ in your example to get something easy to work with, but which isn't a field :-) ]
$endgroup$
– user1729
12 hours ago
add a comment |
5 Answers
5
active
oldest
votes
$begingroup$
The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.
$endgroup$
7
$begingroup$
It's funny because the groups aren't simple.
$endgroup$
– Servaes
12 hours ago
add a comment |
$begingroup$
This example works indeed, if $F$ is infinite and $2^nneq1$ in $F$ for all non-zero $ninBbb{Z}$. This is satisfied for obvious candidates for $F$ such as $Bbb{R}$, $Bbb{C}$ and $Bbb{Q}$, but fails for other candidates such as the finite fields $Bbb{F}_q$, but also infinite fields of positive characteristic such as $Bbb{F}_p(T)$.
Assuming $F$ is a field, the condition that $2^nneq1$ for all non-zero $ninBbb{Z}$ is equivalent to $operatorname{char}F=0$, from which it follows that $F$ is infinite. So your example works if and only if $operatorname{char}F=0$.
$endgroup$
add a comment |
$begingroup$
Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!
It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$
$endgroup$
add a comment |
$begingroup$
A simple example: let $G = S(mathbb Z)$, the group of all permutations of the integers. Let $A$ be the subgroup generated by the transpositions ${(n,n+1) |, n in mathbb Z, n , rm{ even} }$. Since the generating transpositions are all pairwise disjoint, they trivially commute with each other.
$endgroup$
add a comment |
$begingroup$
Yes it works if you take $F$ to be an infinite field for example.
Although, as was pointed out by others even in this case you'll have to make some assumptions on $F$ to get your particular example working.
I guess it'd be more natural to consider the subset of all diagonal submatrices. It certainly is a subgroup as $mathrm{diag}(x,y)^{-1} = mathrm{diag}(x^{-1}, y^{-1})$.
This subgroup is isomorphic to $F^times oplus F^times$, which is abelian and infinite if $F$ is.
$endgroup$
$begingroup$
It does not work for infinite fields such as $Bbb{F}_p(T)$.
$endgroup$
– Servaes
12 hours ago
$begingroup$
ah obviously yes.
$endgroup$
– lush
12 hours ago
$begingroup$
Changed it @Servaes, I had forgotten that he asked for a particular example to work.
$endgroup$
– lush
12 hours ago
add a comment |
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5 Answers
5
active
oldest
votes
5 Answers
5
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.
$endgroup$
7
$begingroup$
It's funny because the groups aren't simple.
$endgroup$
– Servaes
12 hours ago
add a comment |
$begingroup$
The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.
$endgroup$
7
$begingroup$
It's funny because the groups aren't simple.
$endgroup$
– Servaes
12 hours ago
add a comment |
$begingroup$
The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.
$endgroup$
The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.
answered 12 hours ago
lhflhf
167k11172404
167k11172404
7
$begingroup$
It's funny because the groups aren't simple.
$endgroup$
– Servaes
12 hours ago
add a comment |
7
$begingroup$
It's funny because the groups aren't simple.
$endgroup$
– Servaes
12 hours ago
7
7
$begingroup$
It's funny because the groups aren't simple.
$endgroup$
– Servaes
12 hours ago
$begingroup$
It's funny because the groups aren't simple.
$endgroup$
– Servaes
12 hours ago
add a comment |
$begingroup$
This example works indeed, if $F$ is infinite and $2^nneq1$ in $F$ for all non-zero $ninBbb{Z}$. This is satisfied for obvious candidates for $F$ such as $Bbb{R}$, $Bbb{C}$ and $Bbb{Q}$, but fails for other candidates such as the finite fields $Bbb{F}_q$, but also infinite fields of positive characteristic such as $Bbb{F}_p(T)$.
Assuming $F$ is a field, the condition that $2^nneq1$ for all non-zero $ninBbb{Z}$ is equivalent to $operatorname{char}F=0$, from which it follows that $F$ is infinite. So your example works if and only if $operatorname{char}F=0$.
$endgroup$
add a comment |
$begingroup$
This example works indeed, if $F$ is infinite and $2^nneq1$ in $F$ for all non-zero $ninBbb{Z}$. This is satisfied for obvious candidates for $F$ such as $Bbb{R}$, $Bbb{C}$ and $Bbb{Q}$, but fails for other candidates such as the finite fields $Bbb{F}_q$, but also infinite fields of positive characteristic such as $Bbb{F}_p(T)$.
Assuming $F$ is a field, the condition that $2^nneq1$ for all non-zero $ninBbb{Z}$ is equivalent to $operatorname{char}F=0$, from which it follows that $F$ is infinite. So your example works if and only if $operatorname{char}F=0$.
$endgroup$
add a comment |
$begingroup$
This example works indeed, if $F$ is infinite and $2^nneq1$ in $F$ for all non-zero $ninBbb{Z}$. This is satisfied for obvious candidates for $F$ such as $Bbb{R}$, $Bbb{C}$ and $Bbb{Q}$, but fails for other candidates such as the finite fields $Bbb{F}_q$, but also infinite fields of positive characteristic such as $Bbb{F}_p(T)$.
Assuming $F$ is a field, the condition that $2^nneq1$ for all non-zero $ninBbb{Z}$ is equivalent to $operatorname{char}F=0$, from which it follows that $F$ is infinite. So your example works if and only if $operatorname{char}F=0$.
$endgroup$
This example works indeed, if $F$ is infinite and $2^nneq1$ in $F$ for all non-zero $ninBbb{Z}$. This is satisfied for obvious candidates for $F$ such as $Bbb{R}$, $Bbb{C}$ and $Bbb{Q}$, but fails for other candidates such as the finite fields $Bbb{F}_q$, but also infinite fields of positive characteristic such as $Bbb{F}_p(T)$.
Assuming $F$ is a field, the condition that $2^nneq1$ for all non-zero $ninBbb{Z}$ is equivalent to $operatorname{char}F=0$, from which it follows that $F$ is infinite. So your example works if and only if $operatorname{char}F=0$.
answered 12 hours ago
ServaesServaes
29.8k342101
29.8k342101
add a comment |
add a comment |
$begingroup$
Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!
It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$
$endgroup$
add a comment |
$begingroup$
Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!
It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$
$endgroup$
add a comment |
$begingroup$
Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!
It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$
$endgroup$
Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!
It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$
edited 12 hours ago
answered 12 hours ago
Cameron BuieCameron Buie
86.4k773161
86.4k773161
add a comment |
add a comment |
$begingroup$
A simple example: let $G = S(mathbb Z)$, the group of all permutations of the integers. Let $A$ be the subgroup generated by the transpositions ${(n,n+1) |, n in mathbb Z, n , rm{ even} }$. Since the generating transpositions are all pairwise disjoint, they trivially commute with each other.
$endgroup$
add a comment |
$begingroup$
A simple example: let $G = S(mathbb Z)$, the group of all permutations of the integers. Let $A$ be the subgroup generated by the transpositions ${(n,n+1) |, n in mathbb Z, n , rm{ even} }$. Since the generating transpositions are all pairwise disjoint, they trivially commute with each other.
$endgroup$
add a comment |
$begingroup$
A simple example: let $G = S(mathbb Z)$, the group of all permutations of the integers. Let $A$ be the subgroup generated by the transpositions ${(n,n+1) |, n in mathbb Z, n , rm{ even} }$. Since the generating transpositions are all pairwise disjoint, they trivially commute with each other.
$endgroup$
A simple example: let $G = S(mathbb Z)$, the group of all permutations of the integers. Let $A$ be the subgroup generated by the transpositions ${(n,n+1) |, n in mathbb Z, n , rm{ even} }$. Since the generating transpositions are all pairwise disjoint, they trivially commute with each other.
answered 9 hours ago
John ColemanJohn Coleman
3,99311224
3,99311224
add a comment |
add a comment |
$begingroup$
Yes it works if you take $F$ to be an infinite field for example.
Although, as was pointed out by others even in this case you'll have to make some assumptions on $F$ to get your particular example working.
I guess it'd be more natural to consider the subset of all diagonal submatrices. It certainly is a subgroup as $mathrm{diag}(x,y)^{-1} = mathrm{diag}(x^{-1}, y^{-1})$.
This subgroup is isomorphic to $F^times oplus F^times$, which is abelian and infinite if $F$ is.
$endgroup$
$begingroup$
It does not work for infinite fields such as $Bbb{F}_p(T)$.
$endgroup$
– Servaes
12 hours ago
$begingroup$
ah obviously yes.
$endgroup$
– lush
12 hours ago
$begingroup$
Changed it @Servaes, I had forgotten that he asked for a particular example to work.
$endgroup$
– lush
12 hours ago
add a comment |
$begingroup$
Yes it works if you take $F$ to be an infinite field for example.
Although, as was pointed out by others even in this case you'll have to make some assumptions on $F$ to get your particular example working.
I guess it'd be more natural to consider the subset of all diagonal submatrices. It certainly is a subgroup as $mathrm{diag}(x,y)^{-1} = mathrm{diag}(x^{-1}, y^{-1})$.
This subgroup is isomorphic to $F^times oplus F^times$, which is abelian and infinite if $F$ is.
$endgroup$
$begingroup$
It does not work for infinite fields such as $Bbb{F}_p(T)$.
$endgroup$
– Servaes
12 hours ago
$begingroup$
ah obviously yes.
$endgroup$
– lush
12 hours ago
$begingroup$
Changed it @Servaes, I had forgotten that he asked for a particular example to work.
$endgroup$
– lush
12 hours ago
add a comment |
$begingroup$
Yes it works if you take $F$ to be an infinite field for example.
Although, as was pointed out by others even in this case you'll have to make some assumptions on $F$ to get your particular example working.
I guess it'd be more natural to consider the subset of all diagonal submatrices. It certainly is a subgroup as $mathrm{diag}(x,y)^{-1} = mathrm{diag}(x^{-1}, y^{-1})$.
This subgroup is isomorphic to $F^times oplus F^times$, which is abelian and infinite if $F$ is.
$endgroup$
Yes it works if you take $F$ to be an infinite field for example.
Although, as was pointed out by others even in this case you'll have to make some assumptions on $F$ to get your particular example working.
I guess it'd be more natural to consider the subset of all diagonal submatrices. It certainly is a subgroup as $mathrm{diag}(x,y)^{-1} = mathrm{diag}(x^{-1}, y^{-1})$.
This subgroup is isomorphic to $F^times oplus F^times$, which is abelian and infinite if $F$ is.
edited 12 hours ago
answered 12 hours ago
lushlush
732115
732115
$begingroup$
It does not work for infinite fields such as $Bbb{F}_p(T)$.
$endgroup$
– Servaes
12 hours ago
$begingroup$
ah obviously yes.
$endgroup$
– lush
12 hours ago
$begingroup$
Changed it @Servaes, I had forgotten that he asked for a particular example to work.
$endgroup$
– lush
12 hours ago
add a comment |
$begingroup$
It does not work for infinite fields such as $Bbb{F}_p(T)$.
$endgroup$
– Servaes
12 hours ago
$begingroup$
ah obviously yes.
$endgroup$
– lush
12 hours ago
$begingroup$
Changed it @Servaes, I had forgotten that he asked for a particular example to work.
$endgroup$
– lush
12 hours ago
$begingroup$
It does not work for infinite fields such as $Bbb{F}_p(T)$.
$endgroup$
– Servaes
12 hours ago
$begingroup$
It does not work for infinite fields such as $Bbb{F}_p(T)$.
$endgroup$
– Servaes
12 hours ago
$begingroup$
ah obviously yes.
$endgroup$
– lush
12 hours ago
$begingroup$
ah obviously yes.
$endgroup$
– lush
12 hours ago
$begingroup$
Changed it @Servaes, I had forgotten that he asked for a particular example to work.
$endgroup$
– lush
12 hours ago
$begingroup$
Changed it @Servaes, I had forgotten that he asked for a particular example to work.
$endgroup$
– lush
12 hours ago
add a comment |
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$begingroup$
If $A$ is an infinite abelian group and $H$ is a finite, non-abelian group then $Atimes H$ works. [Also, you could take $F=mathbb{Z}$ in your example to get something easy to work with, but which isn't a field :-) ]
$endgroup$
– user1729
12 hours ago