Infinite Abelian subgroup of infinite non Abelian group example












8












$begingroup$


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!










share|cite|improve this question











$endgroup$








  • 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


















8












$begingroup$


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!










share|cite|improve this question











$endgroup$








  • 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
















8












8








8





$begingroup$


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!










share|cite|improve this question











$endgroup$




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






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








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
















  • 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












5 Answers
5






active

oldest

votes


















9












$begingroup$

The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.






share|cite|improve this answer









$endgroup$









  • 7




    $begingroup$
    It's funny because the groups aren't simple.
    $endgroup$
    – Servaes
    12 hours ago



















7












$begingroup$

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 $operatorname{char}F=0$.






share|cite|improve this answer









$endgroup$





















    4












    $begingroup$

    Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!



    It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$






    share|cite|improve this answer











    $endgroup$





















      1












      $begingroup$

      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.






      share|cite|improve this answer









      $endgroup$





















        -1












        $begingroup$

        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.






        share|cite|improve this answer











        $endgroup$













        • $begingroup$
          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












        Your Answer





        StackExchange.ifUsing("editor", function () {
        return StackExchange.using("mathjaxEditing", function () {
        StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
        StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
        });
        });
        }, "mathjax-editing");

        StackExchange.ready(function() {
        var channelOptions = {
        tags: "".split(" "),
        id: "69"
        };
        initTagRenderer("".split(" "), "".split(" "), channelOptions);

        StackExchange.using("externalEditor", function() {
        // Have to fire editor after snippets, if snippets enabled
        if (StackExchange.settings.snippets.snippetsEnabled) {
        StackExchange.using("snippets", function() {
        createEditor();
        });
        }
        else {
        createEditor();
        }
        });

        function createEditor() {
        StackExchange.prepareEditor({
        heartbeatType: 'answer',
        autoActivateHeartbeat: false,
        convertImagesToLinks: true,
        noModals: true,
        showLowRepImageUploadWarning: true,
        reputationToPostImages: 10,
        bindNavPrevention: true,
        postfix: "",
        imageUploader: {
        brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
        contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
        allowUrls: true
        },
        noCode: true, onDemand: true,
        discardSelector: ".discard-answer"
        ,immediatelyShowMarkdownHelp:true
        });


        }
        });














        draft saved

        draft discarded


















        StackExchange.ready(
        function () {
        StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3174561%2finfinite-abelian-subgroup-of-infinite-non-abelian-group-example%23new-answer', 'question_page');
        }
        );

        Post as a guest















        Required, but never shown

























        5 Answers
        5






        active

        oldest

        votes








        5 Answers
        5






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        9












        $begingroup$

        The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.






        share|cite|improve this answer









        $endgroup$









        • 7




          $begingroup$
          It's funny because the groups aren't simple.
          $endgroup$
          – Servaes
          12 hours ago
















        9












        $begingroup$

        The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.






        share|cite|improve this answer









        $endgroup$









        • 7




          $begingroup$
          It's funny because the groups aren't simple.
          $endgroup$
          – Servaes
          12 hours ago














        9












        9








        9





        $begingroup$

        The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.






        share|cite|improve this answer









        $endgroup$



        The simplest example is $G=mathbb Z times S_3$ and $H=mathbb Z$.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered 12 hours ago









        lhflhf

        167k11172404




        167k11172404








        • 7




          $begingroup$
          It's funny because the groups aren't simple.
          $endgroup$
          – Servaes
          12 hours ago














        • 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











        7












        $begingroup$

        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 $operatorname{char}F=0$.






        share|cite|improve this answer









        $endgroup$


















          7












          $begingroup$

          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 $operatorname{char}F=0$.






          share|cite|improve this answer









          $endgroup$
















            7












            7








            7





            $begingroup$

            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 $operatorname{char}F=0$.






            share|cite|improve this answer









            $endgroup$



            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 $operatorname{char}F=0$.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered 12 hours ago









            ServaesServaes

            29.8k342101




            29.8k342101























                4












                $begingroup$

                Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!



                It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$






                share|cite|improve this answer











                $endgroup$


















                  4












                  $begingroup$

                  Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!



                  It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$






                  share|cite|improve this answer











                  $endgroup$
















                    4












                    4








                    4





                    $begingroup$

                    Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!



                    It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$






                    share|cite|improve this answer











                    $endgroup$



                    Assuming that $Bbb F$ has characteristic $0,$ that definitely works. Nicely done!



                    It also allows you to prove an inclusion $Bbb Zhookrightarrow GL(2,Bbb F).$







                    share|cite|improve this answer














                    share|cite|improve this answer



                    share|cite|improve this answer








                    edited 12 hours ago

























                    answered 12 hours ago









                    Cameron BuieCameron Buie

                    86.4k773161




                    86.4k773161























                        1












                        $begingroup$

                        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.






                        share|cite|improve this answer









                        $endgroup$


















                          1












                          $begingroup$

                          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.






                          share|cite|improve this answer









                          $endgroup$
















                            1












                            1








                            1





                            $begingroup$

                            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.






                            share|cite|improve this answer









                            $endgroup$



                            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.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered 9 hours ago









                            John ColemanJohn Coleman

                            3,99311224




                            3,99311224























                                -1












                                $begingroup$

                                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.






                                share|cite|improve this answer











                                $endgroup$













                                • $begingroup$
                                  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
















                                -1












                                $begingroup$

                                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.






                                share|cite|improve this answer











                                $endgroup$













                                • $begingroup$
                                  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














                                -1












                                -1








                                -1





                                $begingroup$

                                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.






                                share|cite|improve this answer











                                $endgroup$



                                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.







                                share|cite|improve this answer














                                share|cite|improve this answer



                                share|cite|improve this answer








                                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


















                                • $begingroup$
                                  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


















                                draft saved

                                draft discarded




















































                                Thanks for contributing an answer to Mathematics Stack Exchange!


                                • Please be sure to answer the question. Provide details and share your research!

                                But avoid



                                • Asking for help, clarification, or responding to other answers.

                                • Making statements based on opinion; back them up with references or personal experience.


                                Use MathJax to format equations. MathJax reference.


                                To learn more, see our tips on writing great answers.




                                draft saved


                                draft discarded














                                StackExchange.ready(
                                function () {
                                StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3174561%2finfinite-abelian-subgroup-of-infinite-non-abelian-group-example%23new-answer', 'question_page');
                                }
                                );

                                Post as a guest















                                Required, but never shown





















































                                Required, but never shown














                                Required, but never shown












                                Required, but never shown







                                Required, but never shown

































                                Required, but never shown














                                Required, but never shown












                                Required, but never shown







                                Required, but never shown







                                Popular posts from this blog

                                香粉寮

                                GameSpot