How many cyclic subgroups are in the Dihedral group?












4












$begingroup$


let $D_8$ be the group of symmetries of a square. Not counting the trivial subgroup, how many distinct cyclic subgroup s does $D_8$ contain?



My result:
A group $D_8$, contains an element $a$ of order 4.



$<{a}> =(a^4:4in Z)$



$(1.a,a^2,a^3,x,b,c,d)$ the reflections and symmetries in $D_8$.



$(1)(a)=a
\
(a)(a)=a^2
\
(a)(a^2)=a^3
\
(a)(a^3)=a^4=1$



$(1,a,a^2,a^3)$



Did I answer this correctly? If yes can someone clarify my writing or if it's wrong please guide me ...










share|cite|improve this question











$endgroup$

















    4












    $begingroup$


    let $D_8$ be the group of symmetries of a square. Not counting the trivial subgroup, how many distinct cyclic subgroup s does $D_8$ contain?



    My result:
    A group $D_8$, contains an element $a$ of order 4.



    $<{a}> =(a^4:4in Z)$



    $(1.a,a^2,a^3,x,b,c,d)$ the reflections and symmetries in $D_8$.



    $(1)(a)=a
    \
    (a)(a)=a^2
    \
    (a)(a^2)=a^3
    \
    (a)(a^3)=a^4=1$



    $(1,a,a^2,a^3)$



    Did I answer this correctly? If yes can someone clarify my writing or if it's wrong please guide me ...










    share|cite|improve this question











    $endgroup$















      4












      4








      4





      $begingroup$


      let $D_8$ be the group of symmetries of a square. Not counting the trivial subgroup, how many distinct cyclic subgroup s does $D_8$ contain?



      My result:
      A group $D_8$, contains an element $a$ of order 4.



      $<{a}> =(a^4:4in Z)$



      $(1.a,a^2,a^3,x,b,c,d)$ the reflections and symmetries in $D_8$.



      $(1)(a)=a
      \
      (a)(a)=a^2
      \
      (a)(a^2)=a^3
      \
      (a)(a^3)=a^4=1$



      $(1,a,a^2,a^3)$



      Did I answer this correctly? If yes can someone clarify my writing or if it's wrong please guide me ...










      share|cite|improve this question











      $endgroup$




      let $D_8$ be the group of symmetries of a square. Not counting the trivial subgroup, how many distinct cyclic subgroup s does $D_8$ contain?



      My result:
      A group $D_8$, contains an element $a$ of order 4.



      $<{a}> =(a^4:4in Z)$



      $(1.a,a^2,a^3,x,b,c,d)$ the reflections and symmetries in $D_8$.



      $(1)(a)=a
      \
      (a)(a)=a^2
      \
      (a)(a^2)=a^3
      \
      (a)(a^3)=a^4=1$



      $(1,a,a^2,a^3)$



      Did I answer this correctly? If yes can someone clarify my writing or if it's wrong please guide me ...







      abstract-algebra group-theory cyclic-groups






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Oct 8 '13 at 15:11









      user1729

      17k64085




      17k64085










      asked Sep 22 '13 at 22:53









      miguel barnesmiguel barnes

      13917




      13917






















          4 Answers
          4






          active

          oldest

          votes


















          5












          $begingroup$

          You found one such cyclic subgroup.



          We also have a cyclic subgroup that's also a subgroup of the cyclic group you found, of order two:$${1, a^2}$$



          Four additional cyclic subgroups of order two are as follows:



          $${1, x}, {1, b}, {1, c}, {1, d}$$



          Of course, we need also to add a sixth, trivially cyclic but distinct subgroup: ${1}$.



          So, with the subgroup you found, and the additional 6 subgroups here, we have, in all, $7$ distinct cyclic subgroups of $D_8$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            How did you obtain those cyclic subgroups? Isn't $(1,x)$ a cyclic subgroup as well?
            $endgroup$
            – miguel barnes
            Sep 23 '13 at 1:56








          • 1




            $begingroup$
            Yes, ${1, x}$ is another cyclic subgroup of order 2. I missed seeing x in your set of elements.
            $endgroup$
            – amWhy
            Sep 23 '13 at 1:58



















          2












          $begingroup$

          Your notation is not very clear, so I don't quite understand your answer. Note that your answer should be a natural number: the number of distinct cyclic subgroups of $D_8$ other than the trivial subgroup.



          To find out what that number is, you can just go over each and every element of $D_8$ and check what it generates. That will give you a list of all the cyclic subgroups. You then identify how many distinct ones there are, and that will be your answer.



          Note that $D_8$ only has $8$ elements so this is certainly easy enough to just do (also note that some texts call your $D_8$ by $D_4$).






          share|cite|improve this answer









          $endgroup$





















            0












            $begingroup$

            Below, we can see the subgroups concrete. It's done by GAP:



            gap> LoadPackage("sonata");;
            f:=FreeGroup("a","b");;
            a:=f.1;; b:=f.2;;
            s:=f/[a^2,b^4,(a*b)^2];
            e:=Subgroups(s);;
            for i in [1..8] do Print(e[i]," ",IsCyclic(e[i]),"n"); od;




               Group( <identity ...> )  true
            Group( [ b^2 ] ) true
            Group( [ a ] ) true
            Group( [ b^-1*a*b ] ) true
            Group( [ a*b ] ) true
            Group( [ b*a ] ) true
            Group( [ a, b^-2 ] ) false
            Group( [ b ] ) true





            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              Length of e is 10, not 8 - any considerations against using more generic for i in [1..Length(e)] syntax?
              $endgroup$
              – Alexander Konovalov
              Oct 8 '13 at 23:24



















            0












            $begingroup$

            No. of cyclic subgroups of $D_n $ ( order of $D_n$ is $2n$) is
            $tau{(n)} +$ $n$. Where $tau(n)$ is number of positive divisor of $n$.
            And total number of subgroups of $D_n$ is $tau(n) + sigma(n)$ where $sigma(n)$ is sum of all positive divisor.






            share|cite|improve this answer











            $endgroup$













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              4 Answers
              4






              active

              oldest

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              4 Answers
              4






              active

              oldest

              votes









              active

              oldest

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              active

              oldest

              votes









              5












              $begingroup$

              You found one such cyclic subgroup.



              We also have a cyclic subgroup that's also a subgroup of the cyclic group you found, of order two:$${1, a^2}$$



              Four additional cyclic subgroups of order two are as follows:



              $${1, x}, {1, b}, {1, c}, {1, d}$$



              Of course, we need also to add a sixth, trivially cyclic but distinct subgroup: ${1}$.



              So, with the subgroup you found, and the additional 6 subgroups here, we have, in all, $7$ distinct cyclic subgroups of $D_8$.






              share|cite|improve this answer











              $endgroup$













              • $begingroup$
                How did you obtain those cyclic subgroups? Isn't $(1,x)$ a cyclic subgroup as well?
                $endgroup$
                – miguel barnes
                Sep 23 '13 at 1:56








              • 1




                $begingroup$
                Yes, ${1, x}$ is another cyclic subgroup of order 2. I missed seeing x in your set of elements.
                $endgroup$
                – amWhy
                Sep 23 '13 at 1:58
















              5












              $begingroup$

              You found one such cyclic subgroup.



              We also have a cyclic subgroup that's also a subgroup of the cyclic group you found, of order two:$${1, a^2}$$



              Four additional cyclic subgroups of order two are as follows:



              $${1, x}, {1, b}, {1, c}, {1, d}$$



              Of course, we need also to add a sixth, trivially cyclic but distinct subgroup: ${1}$.



              So, with the subgroup you found, and the additional 6 subgroups here, we have, in all, $7$ distinct cyclic subgroups of $D_8$.






              share|cite|improve this answer











              $endgroup$













              • $begingroup$
                How did you obtain those cyclic subgroups? Isn't $(1,x)$ a cyclic subgroup as well?
                $endgroup$
                – miguel barnes
                Sep 23 '13 at 1:56








              • 1




                $begingroup$
                Yes, ${1, x}$ is another cyclic subgroup of order 2. I missed seeing x in your set of elements.
                $endgroup$
                – amWhy
                Sep 23 '13 at 1:58














              5












              5








              5





              $begingroup$

              You found one such cyclic subgroup.



              We also have a cyclic subgroup that's also a subgroup of the cyclic group you found, of order two:$${1, a^2}$$



              Four additional cyclic subgroups of order two are as follows:



              $${1, x}, {1, b}, {1, c}, {1, d}$$



              Of course, we need also to add a sixth, trivially cyclic but distinct subgroup: ${1}$.



              So, with the subgroup you found, and the additional 6 subgroups here, we have, in all, $7$ distinct cyclic subgroups of $D_8$.






              share|cite|improve this answer











              $endgroup$



              You found one such cyclic subgroup.



              We also have a cyclic subgroup that's also a subgroup of the cyclic group you found, of order two:$${1, a^2}$$



              Four additional cyclic subgroups of order two are as follows:



              $${1, x}, {1, b}, {1, c}, {1, d}$$



              Of course, we need also to add a sixth, trivially cyclic but distinct subgroup: ${1}$.



              So, with the subgroup you found, and the additional 6 subgroups here, we have, in all, $7$ distinct cyclic subgroups of $D_8$.







              share|cite|improve this answer














              share|cite|improve this answer



              share|cite|improve this answer








              edited Sep 23 '13 at 1:59

























              answered Sep 22 '13 at 23:01









              amWhyamWhy

              1




              1












              • $begingroup$
                How did you obtain those cyclic subgroups? Isn't $(1,x)$ a cyclic subgroup as well?
                $endgroup$
                – miguel barnes
                Sep 23 '13 at 1:56








              • 1




                $begingroup$
                Yes, ${1, x}$ is another cyclic subgroup of order 2. I missed seeing x in your set of elements.
                $endgroup$
                – amWhy
                Sep 23 '13 at 1:58


















              • $begingroup$
                How did you obtain those cyclic subgroups? Isn't $(1,x)$ a cyclic subgroup as well?
                $endgroup$
                – miguel barnes
                Sep 23 '13 at 1:56








              • 1




                $begingroup$
                Yes, ${1, x}$ is another cyclic subgroup of order 2. I missed seeing x in your set of elements.
                $endgroup$
                – amWhy
                Sep 23 '13 at 1:58
















              $begingroup$
              How did you obtain those cyclic subgroups? Isn't $(1,x)$ a cyclic subgroup as well?
              $endgroup$
              – miguel barnes
              Sep 23 '13 at 1:56






              $begingroup$
              How did you obtain those cyclic subgroups? Isn't $(1,x)$ a cyclic subgroup as well?
              $endgroup$
              – miguel barnes
              Sep 23 '13 at 1:56






              1




              1




              $begingroup$
              Yes, ${1, x}$ is another cyclic subgroup of order 2. I missed seeing x in your set of elements.
              $endgroup$
              – amWhy
              Sep 23 '13 at 1:58




              $begingroup$
              Yes, ${1, x}$ is another cyclic subgroup of order 2. I missed seeing x in your set of elements.
              $endgroup$
              – amWhy
              Sep 23 '13 at 1:58











              2












              $begingroup$

              Your notation is not very clear, so I don't quite understand your answer. Note that your answer should be a natural number: the number of distinct cyclic subgroups of $D_8$ other than the trivial subgroup.



              To find out what that number is, you can just go over each and every element of $D_8$ and check what it generates. That will give you a list of all the cyclic subgroups. You then identify how many distinct ones there are, and that will be your answer.



              Note that $D_8$ only has $8$ elements so this is certainly easy enough to just do (also note that some texts call your $D_8$ by $D_4$).






              share|cite|improve this answer









              $endgroup$


















                2












                $begingroup$

                Your notation is not very clear, so I don't quite understand your answer. Note that your answer should be a natural number: the number of distinct cyclic subgroups of $D_8$ other than the trivial subgroup.



                To find out what that number is, you can just go over each and every element of $D_8$ and check what it generates. That will give you a list of all the cyclic subgroups. You then identify how many distinct ones there are, and that will be your answer.



                Note that $D_8$ only has $8$ elements so this is certainly easy enough to just do (also note that some texts call your $D_8$ by $D_4$).






                share|cite|improve this answer









                $endgroup$
















                  2












                  2








                  2





                  $begingroup$

                  Your notation is not very clear, so I don't quite understand your answer. Note that your answer should be a natural number: the number of distinct cyclic subgroups of $D_8$ other than the trivial subgroup.



                  To find out what that number is, you can just go over each and every element of $D_8$ and check what it generates. That will give you a list of all the cyclic subgroups. You then identify how many distinct ones there are, and that will be your answer.



                  Note that $D_8$ only has $8$ elements so this is certainly easy enough to just do (also note that some texts call your $D_8$ by $D_4$).






                  share|cite|improve this answer









                  $endgroup$



                  Your notation is not very clear, so I don't quite understand your answer. Note that your answer should be a natural number: the number of distinct cyclic subgroups of $D_8$ other than the trivial subgroup.



                  To find out what that number is, you can just go over each and every element of $D_8$ and check what it generates. That will give you a list of all the cyclic subgroups. You then identify how many distinct ones there are, and that will be your answer.



                  Note that $D_8$ only has $8$ elements so this is certainly easy enough to just do (also note that some texts call your $D_8$ by $D_4$).







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Sep 22 '13 at 23:02









                  Ittay WeissIttay Weiss

                  63.8k6101184




                  63.8k6101184























                      0












                      $begingroup$

                      Below, we can see the subgroups concrete. It's done by GAP:



                      gap> LoadPackage("sonata");;
                      f:=FreeGroup("a","b");;
                      a:=f.1;; b:=f.2;;
                      s:=f/[a^2,b^4,(a*b)^2];
                      e:=Subgroups(s);;
                      for i in [1..8] do Print(e[i]," ",IsCyclic(e[i]),"n"); od;




                         Group( <identity ...> )  true
                      Group( [ b^2 ] ) true
                      Group( [ a ] ) true
                      Group( [ b^-1*a*b ] ) true
                      Group( [ a*b ] ) true
                      Group( [ b*a ] ) true
                      Group( [ a, b^-2 ] ) false
                      Group( [ b ] ) true





                      share|cite|improve this answer











                      $endgroup$













                      • $begingroup$
                        Length of e is 10, not 8 - any considerations against using more generic for i in [1..Length(e)] syntax?
                        $endgroup$
                        – Alexander Konovalov
                        Oct 8 '13 at 23:24
















                      0












                      $begingroup$

                      Below, we can see the subgroups concrete. It's done by GAP:



                      gap> LoadPackage("sonata");;
                      f:=FreeGroup("a","b");;
                      a:=f.1;; b:=f.2;;
                      s:=f/[a^2,b^4,(a*b)^2];
                      e:=Subgroups(s);;
                      for i in [1..8] do Print(e[i]," ",IsCyclic(e[i]),"n"); od;




                         Group( <identity ...> )  true
                      Group( [ b^2 ] ) true
                      Group( [ a ] ) true
                      Group( [ b^-1*a*b ] ) true
                      Group( [ a*b ] ) true
                      Group( [ b*a ] ) true
                      Group( [ a, b^-2 ] ) false
                      Group( [ b ] ) true





                      share|cite|improve this answer











                      $endgroup$













                      • $begingroup$
                        Length of e is 10, not 8 - any considerations against using more generic for i in [1..Length(e)] syntax?
                        $endgroup$
                        – Alexander Konovalov
                        Oct 8 '13 at 23:24














                      0












                      0








                      0





                      $begingroup$

                      Below, we can see the subgroups concrete. It's done by GAP:



                      gap> LoadPackage("sonata");;
                      f:=FreeGroup("a","b");;
                      a:=f.1;; b:=f.2;;
                      s:=f/[a^2,b^4,(a*b)^2];
                      e:=Subgroups(s);;
                      for i in [1..8] do Print(e[i]," ",IsCyclic(e[i]),"n"); od;




                         Group( <identity ...> )  true
                      Group( [ b^2 ] ) true
                      Group( [ a ] ) true
                      Group( [ b^-1*a*b ] ) true
                      Group( [ a*b ] ) true
                      Group( [ b*a ] ) true
                      Group( [ a, b^-2 ] ) false
                      Group( [ b ] ) true





                      share|cite|improve this answer











                      $endgroup$



                      Below, we can see the subgroups concrete. It's done by GAP:



                      gap> LoadPackage("sonata");;
                      f:=FreeGroup("a","b");;
                      a:=f.1;; b:=f.2;;
                      s:=f/[a^2,b^4,(a*b)^2];
                      e:=Subgroups(s);;
                      for i in [1..8] do Print(e[i]," ",IsCyclic(e[i]),"n"); od;




                         Group( <identity ...> )  true
                      Group( [ b^2 ] ) true
                      Group( [ a ] ) true
                      Group( [ b^-1*a*b ] ) true
                      Group( [ a*b ] ) true
                      Group( [ b*a ] ) true
                      Group( [ a, b^-2 ] ) false
                      Group( [ b ] ) true






                      share|cite|improve this answer














                      share|cite|improve this answer



                      share|cite|improve this answer








                      edited Oct 8 '13 at 15:10

























                      answered Oct 8 '13 at 15:04









                      mrsmrs

                      1




                      1












                      • $begingroup$
                        Length of e is 10, not 8 - any considerations against using more generic for i in [1..Length(e)] syntax?
                        $endgroup$
                        – Alexander Konovalov
                        Oct 8 '13 at 23:24


















                      • $begingroup$
                        Length of e is 10, not 8 - any considerations against using more generic for i in [1..Length(e)] syntax?
                        $endgroup$
                        – Alexander Konovalov
                        Oct 8 '13 at 23:24
















                      $begingroup$
                      Length of e is 10, not 8 - any considerations against using more generic for i in [1..Length(e)] syntax?
                      $endgroup$
                      – Alexander Konovalov
                      Oct 8 '13 at 23:24




                      $begingroup$
                      Length of e is 10, not 8 - any considerations against using more generic for i in [1..Length(e)] syntax?
                      $endgroup$
                      – Alexander Konovalov
                      Oct 8 '13 at 23:24











                      0












                      $begingroup$

                      No. of cyclic subgroups of $D_n $ ( order of $D_n$ is $2n$) is
                      $tau{(n)} +$ $n$. Where $tau(n)$ is number of positive divisor of $n$.
                      And total number of subgroups of $D_n$ is $tau(n) + sigma(n)$ where $sigma(n)$ is sum of all positive divisor.






                      share|cite|improve this answer











                      $endgroup$


















                        0












                        $begingroup$

                        No. of cyclic subgroups of $D_n $ ( order of $D_n$ is $2n$) is
                        $tau{(n)} +$ $n$. Where $tau(n)$ is number of positive divisor of $n$.
                        And total number of subgroups of $D_n$ is $tau(n) + sigma(n)$ where $sigma(n)$ is sum of all positive divisor.






                        share|cite|improve this answer











                        $endgroup$
















                          0












                          0








                          0





                          $begingroup$

                          No. of cyclic subgroups of $D_n $ ( order of $D_n$ is $2n$) is
                          $tau{(n)} +$ $n$. Where $tau(n)$ is number of positive divisor of $n$.
                          And total number of subgroups of $D_n$ is $tau(n) + sigma(n)$ where $sigma(n)$ is sum of all positive divisor.






                          share|cite|improve this answer











                          $endgroup$



                          No. of cyclic subgroups of $D_n $ ( order of $D_n$ is $2n$) is
                          $tau{(n)} +$ $n$. Where $tau(n)$ is number of positive divisor of $n$.
                          And total number of subgroups of $D_n$ is $tau(n) + sigma(n)$ where $sigma(n)$ is sum of all positive divisor.







                          share|cite|improve this answer














                          share|cite|improve this answer



                          share|cite|improve this answer








                          edited Dec 11 '18 at 9:39

























                          answered Dec 11 '18 at 9:34









                          August ariesAugust aries

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