How to find the order of a symmetric group S4?












2












$begingroup$


Let $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$
and
$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$, both elements of $S_4$.



How exactly do I compute the orders of $omega$ and $tau$ ?



My professor said $ |omega|= 3$ and$ |tau|= 4$.
But he did not explain how to get these answers, also why are they both not just 4? I thought order was the number of elements in G, in this case they both have 4.
Does $ |omega|= 3$ because $omega(2)=2$, so it sends itself to itself?



Sorry, if this question seems dumb I just don't fully understand where the answer is coming from.










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$endgroup$








  • 1




    $begingroup$
    Try compute $omega^3$, $tau^2$, $tau^4$ to get the order of these two elements.
    $endgroup$
    – Kemono Chen
    5 hours ago










  • $begingroup$
    It's not exactly about the number of fixed points. For example, $sigma= bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 &1 & 4 & 3 end{smallmatrix}bigr)$ has order 2. Can you see why, now that the correct definition of "order" in this context has been explained?
    $endgroup$
    – Robert Shore
    4 hours ago
















2












$begingroup$


Let $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$
and
$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$, both elements of $S_4$.



How exactly do I compute the orders of $omega$ and $tau$ ?



My professor said $ |omega|= 3$ and$ |tau|= 4$.
But he did not explain how to get these answers, also why are they both not just 4? I thought order was the number of elements in G, in this case they both have 4.
Does $ |omega|= 3$ because $omega(2)=2$, so it sends itself to itself?



Sorry, if this question seems dumb I just don't fully understand where the answer is coming from.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Try compute $omega^3$, $tau^2$, $tau^4$ to get the order of these two elements.
    $endgroup$
    – Kemono Chen
    5 hours ago










  • $begingroup$
    It's not exactly about the number of fixed points. For example, $sigma= bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 &1 & 4 & 3 end{smallmatrix}bigr)$ has order 2. Can you see why, now that the correct definition of "order" in this context has been explained?
    $endgroup$
    – Robert Shore
    4 hours ago














2












2








2





$begingroup$


Let $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$
and
$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$, both elements of $S_4$.



How exactly do I compute the orders of $omega$ and $tau$ ?



My professor said $ |omega|= 3$ and$ |tau|= 4$.
But he did not explain how to get these answers, also why are they both not just 4? I thought order was the number of elements in G, in this case they both have 4.
Does $ |omega|= 3$ because $omega(2)=2$, so it sends itself to itself?



Sorry, if this question seems dumb I just don't fully understand where the answer is coming from.










share|cite|improve this question











$endgroup$




Let $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$
and
$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$, both elements of $S_4$.



How exactly do I compute the orders of $omega$ and $tau$ ?



My professor said $ |omega|= 3$ and$ |tau|= 4$.
But he did not explain how to get these answers, also why are they both not just 4? I thought order was the number of elements in G, in this case they both have 4.
Does $ |omega|= 3$ because $omega(2)=2$, so it sends itself to itself?



Sorry, if this question seems dumb I just don't fully understand where the answer is coming from.







abstract-algebra group-theory permutations






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share|cite|improve this question













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share|cite|improve this question








edited 5 hours ago









Kemono Chen

3,1641844




3,1641844










asked 5 hours ago









User100290392039User100290392039

435




435








  • 1




    $begingroup$
    Try compute $omega^3$, $tau^2$, $tau^4$ to get the order of these two elements.
    $endgroup$
    – Kemono Chen
    5 hours ago










  • $begingroup$
    It's not exactly about the number of fixed points. For example, $sigma= bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 &1 & 4 & 3 end{smallmatrix}bigr)$ has order 2. Can you see why, now that the correct definition of "order" in this context has been explained?
    $endgroup$
    – Robert Shore
    4 hours ago














  • 1




    $begingroup$
    Try compute $omega^3$, $tau^2$, $tau^4$ to get the order of these two elements.
    $endgroup$
    – Kemono Chen
    5 hours ago










  • $begingroup$
    It's not exactly about the number of fixed points. For example, $sigma= bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 &1 & 4 & 3 end{smallmatrix}bigr)$ has order 2. Can you see why, now that the correct definition of "order" in this context has been explained?
    $endgroup$
    – Robert Shore
    4 hours ago








1




1




$begingroup$
Try compute $omega^3$, $tau^2$, $tau^4$ to get the order of these two elements.
$endgroup$
– Kemono Chen
5 hours ago




$begingroup$
Try compute $omega^3$, $tau^2$, $tau^4$ to get the order of these two elements.
$endgroup$
– Kemono Chen
5 hours ago












$begingroup$
It's not exactly about the number of fixed points. For example, $sigma= bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 &1 & 4 & 3 end{smallmatrix}bigr)$ has order 2. Can you see why, now that the correct definition of "order" in this context has been explained?
$endgroup$
– Robert Shore
4 hours ago




$begingroup$
It's not exactly about the number of fixed points. For example, $sigma= bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 &1 & 4 & 3 end{smallmatrix}bigr)$ has order 2. Can you see why, now that the correct definition of "order" in this context has been explained?
$endgroup$
– Robert Shore
4 hours ago










4 Answers
4






active

oldest

votes


















2












$begingroup$

The order of a group is not the same thing as the order of an element. There is a connection, but don't worry about that yet. The order of a group, written $|G|$, is the number of elements it has. The order of an element in a group - say $g in G$, is the smallest $n$ such $g^n = e$ where $e$ is the identity element of the group $G$. In this case we also write $|g| = n$.



You are not being asked to find the order of a group. $tau$ and $omega$ are elements of a group and you are being asked to find the order of $tau$ and $omega$ as elements of $S_4$. Elements of $S_4$, such as $tau$ and $omega$, are bijections (functions) from the set ${1,2,3,4}$ to the set ${1,2,3,4}$. To find their order you need to investigate how many times you need to repeatedly apply the function in order to get the identity function. It doesn't really make sense to take about the order of $tau$ and $omega$ based on how many elements they have as you did in your original post, however we do have this.



It does turn out that if the order of an element is $n$, say $|g| = n$, then the cyclic subgroup generated by $g$ will have $n$ elements. So I guess if you wanted to find the orders of $tau$ and $omega$ you could go count the elements of their cyclic subgroups but this seems superfluous.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    If we want to find the order of $S_4$, it should be $4!$, right?
    $endgroup$
    – manooooh
    5 hours ago






  • 2




    $begingroup$
    Yes, but if you read the question you will realize that the title of the question is not really what the OP is asking.
    $endgroup$
    – Prince M
    5 hours ago





















2












$begingroup$

We first need to know that when talking about $S_4$, we are talking about the set of all bijections from the set ${1,2,3,4}$ to itself.



So, the elements of the set $S_4$ are functions. An example of a function in $S_4$ is the function $f: {1,2,3,4} to {1,2,3,4}$ given by:
$$1 mapsto 2, 2mapsto 3, 3mapsto 4, 4 mapsto 1 $$



However, this is incredibly tedious to write out. Instead, we will use matrices to represent functions in $S_4$. In other words, we will let:



$$f := bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$$
where an element in row 1 of the matrix is mapped to the element below it in row 2 of the matrix by $f$.



Now, when we are talking about order, we can talk about 2 things: the order of a group, and the order of an element of the group.



The order of a group is the number of elements in that set, but we can also pick out an element from that group (in this case the group is $S_4$ and we're picking out an element from $S_4$) and we can ask what is the order of the element. The order of an element is the smallest positive number $n$ such that $x^n = 1$ where $x$ is the element of the group we are considering, and $1$ is the identity element in the group.



In your case, when your professor says that $|omega| = 3$, what he means is that if you compose the function $omega in S_4$ with itself 3 times, you will get back the identity element of $S_4$. The identity element of $S_4$ is the identity function on ${1,2,3,4}$, i.e. the function:



$$i = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr)$$



For instance, although $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$, we see that $omega^2 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 4 & 2 & 1 & 3 end{smallmatrix}bigr)$, and $omega^3 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr) = i$






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$endgroup$





















    0












    $begingroup$

    In "cycle notation ", $omega=(134)$ is a $3$-cycle, and $tau=(1234)$ is a $4$-cycle.



    It happens that $n$-cycles always have order $n$.






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      We may always write a permutation as 1 or more cyclic permutations:



      For example, for $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$ you may trace the path of element $1$ and see that



      $$1 mapsto 3 mapsto 4 mapsto 1$$



      is a cyclic permutation of 3 elements:
      enter image description here



      Tracing the remaining element $2$ we see that



      $$2 mapsto 2$$



      which is identity of 1-element set, considered as a cyclic permutation, too.



      Using the notation for cyclic permutations, the first one is



      $$(1 3 4)$$



      and the second one is simply $$(2)$$



      As the result, we may write



      $$omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)= (1 3 4) (2)$$



      Now, the first, 3-element cyclic permutation
      has the order $mathbb 3$ (its "cycle" must turn 3-times $120^o$ to return to its initial position)
      enter image description here



      and the the order of the second one is $mathbb 1$ (order of identity). Their least common multiple is $lcm(3, 1)=3$, so



      $$ord(omega) = 3$$





      For $tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$ is the situation very simple, because it is a cyclic one :



      $$1 mapsto 2 mapsto 3 mapsto 4 mapsto 1$$



      so we may write



      $$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr) = (1 2 3 4)$$



      and



      $$ord(tau) = 4$$






      share|cite|improve this answer











      $endgroup$













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        4 Answers
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        4 Answers
        4






        active

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        active

        oldest

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        2












        $begingroup$

        The order of a group is not the same thing as the order of an element. There is a connection, but don't worry about that yet. The order of a group, written $|G|$, is the number of elements it has. The order of an element in a group - say $g in G$, is the smallest $n$ such $g^n = e$ where $e$ is the identity element of the group $G$. In this case we also write $|g| = n$.



        You are not being asked to find the order of a group. $tau$ and $omega$ are elements of a group and you are being asked to find the order of $tau$ and $omega$ as elements of $S_4$. Elements of $S_4$, such as $tau$ and $omega$, are bijections (functions) from the set ${1,2,3,4}$ to the set ${1,2,3,4}$. To find their order you need to investigate how many times you need to repeatedly apply the function in order to get the identity function. It doesn't really make sense to take about the order of $tau$ and $omega$ based on how many elements they have as you did in your original post, however we do have this.



        It does turn out that if the order of an element is $n$, say $|g| = n$, then the cyclic subgroup generated by $g$ will have $n$ elements. So I guess if you wanted to find the orders of $tau$ and $omega$ you could go count the elements of their cyclic subgroups but this seems superfluous.






        share|cite|improve this answer











        $endgroup$













        • $begingroup$
          If we want to find the order of $S_4$, it should be $4!$, right?
          $endgroup$
          – manooooh
          5 hours ago






        • 2




          $begingroup$
          Yes, but if you read the question you will realize that the title of the question is not really what the OP is asking.
          $endgroup$
          – Prince M
          5 hours ago


















        2












        $begingroup$

        The order of a group is not the same thing as the order of an element. There is a connection, but don't worry about that yet. The order of a group, written $|G|$, is the number of elements it has. The order of an element in a group - say $g in G$, is the smallest $n$ such $g^n = e$ where $e$ is the identity element of the group $G$. In this case we also write $|g| = n$.



        You are not being asked to find the order of a group. $tau$ and $omega$ are elements of a group and you are being asked to find the order of $tau$ and $omega$ as elements of $S_4$. Elements of $S_4$, such as $tau$ and $omega$, are bijections (functions) from the set ${1,2,3,4}$ to the set ${1,2,3,4}$. To find their order you need to investigate how many times you need to repeatedly apply the function in order to get the identity function. It doesn't really make sense to take about the order of $tau$ and $omega$ based on how many elements they have as you did in your original post, however we do have this.



        It does turn out that if the order of an element is $n$, say $|g| = n$, then the cyclic subgroup generated by $g$ will have $n$ elements. So I guess if you wanted to find the orders of $tau$ and $omega$ you could go count the elements of their cyclic subgroups but this seems superfluous.






        share|cite|improve this answer











        $endgroup$













        • $begingroup$
          If we want to find the order of $S_4$, it should be $4!$, right?
          $endgroup$
          – manooooh
          5 hours ago






        • 2




          $begingroup$
          Yes, but if you read the question you will realize that the title of the question is not really what the OP is asking.
          $endgroup$
          – Prince M
          5 hours ago
















        2












        2








        2





        $begingroup$

        The order of a group is not the same thing as the order of an element. There is a connection, but don't worry about that yet. The order of a group, written $|G|$, is the number of elements it has. The order of an element in a group - say $g in G$, is the smallest $n$ such $g^n = e$ where $e$ is the identity element of the group $G$. In this case we also write $|g| = n$.



        You are not being asked to find the order of a group. $tau$ and $omega$ are elements of a group and you are being asked to find the order of $tau$ and $omega$ as elements of $S_4$. Elements of $S_4$, such as $tau$ and $omega$, are bijections (functions) from the set ${1,2,3,4}$ to the set ${1,2,3,4}$. To find their order you need to investigate how many times you need to repeatedly apply the function in order to get the identity function. It doesn't really make sense to take about the order of $tau$ and $omega$ based on how many elements they have as you did in your original post, however we do have this.



        It does turn out that if the order of an element is $n$, say $|g| = n$, then the cyclic subgroup generated by $g$ will have $n$ elements. So I guess if you wanted to find the orders of $tau$ and $omega$ you could go count the elements of their cyclic subgroups but this seems superfluous.






        share|cite|improve this answer











        $endgroup$



        The order of a group is not the same thing as the order of an element. There is a connection, but don't worry about that yet. The order of a group, written $|G|$, is the number of elements it has. The order of an element in a group - say $g in G$, is the smallest $n$ such $g^n = e$ where $e$ is the identity element of the group $G$. In this case we also write $|g| = n$.



        You are not being asked to find the order of a group. $tau$ and $omega$ are elements of a group and you are being asked to find the order of $tau$ and $omega$ as elements of $S_4$. Elements of $S_4$, such as $tau$ and $omega$, are bijections (functions) from the set ${1,2,3,4}$ to the set ${1,2,3,4}$. To find their order you need to investigate how many times you need to repeatedly apply the function in order to get the identity function. It doesn't really make sense to take about the order of $tau$ and $omega$ based on how many elements they have as you did in your original post, however we do have this.



        It does turn out that if the order of an element is $n$, say $|g| = n$, then the cyclic subgroup generated by $g$ will have $n$ elements. So I guess if you wanted to find the orders of $tau$ and $omega$ you could go count the elements of their cyclic subgroups but this seems superfluous.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited 5 hours ago

























        answered 5 hours ago









        Prince MPrince M

        2,0531521




        2,0531521












        • $begingroup$
          If we want to find the order of $S_4$, it should be $4!$, right?
          $endgroup$
          – manooooh
          5 hours ago






        • 2




          $begingroup$
          Yes, but if you read the question you will realize that the title of the question is not really what the OP is asking.
          $endgroup$
          – Prince M
          5 hours ago




















        • $begingroup$
          If we want to find the order of $S_4$, it should be $4!$, right?
          $endgroup$
          – manooooh
          5 hours ago






        • 2




          $begingroup$
          Yes, but if you read the question you will realize that the title of the question is not really what the OP is asking.
          $endgroup$
          – Prince M
          5 hours ago


















        $begingroup$
        If we want to find the order of $S_4$, it should be $4!$, right?
        $endgroup$
        – manooooh
        5 hours ago




        $begingroup$
        If we want to find the order of $S_4$, it should be $4!$, right?
        $endgroup$
        – manooooh
        5 hours ago




        2




        2




        $begingroup$
        Yes, but if you read the question you will realize that the title of the question is not really what the OP is asking.
        $endgroup$
        – Prince M
        5 hours ago






        $begingroup$
        Yes, but if you read the question you will realize that the title of the question is not really what the OP is asking.
        $endgroup$
        – Prince M
        5 hours ago













        2












        $begingroup$

        We first need to know that when talking about $S_4$, we are talking about the set of all bijections from the set ${1,2,3,4}$ to itself.



        So, the elements of the set $S_4$ are functions. An example of a function in $S_4$ is the function $f: {1,2,3,4} to {1,2,3,4}$ given by:
        $$1 mapsto 2, 2mapsto 3, 3mapsto 4, 4 mapsto 1 $$



        However, this is incredibly tedious to write out. Instead, we will use matrices to represent functions in $S_4$. In other words, we will let:



        $$f := bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$$
        where an element in row 1 of the matrix is mapped to the element below it in row 2 of the matrix by $f$.



        Now, when we are talking about order, we can talk about 2 things: the order of a group, and the order of an element of the group.



        The order of a group is the number of elements in that set, but we can also pick out an element from that group (in this case the group is $S_4$ and we're picking out an element from $S_4$) and we can ask what is the order of the element. The order of an element is the smallest positive number $n$ such that $x^n = 1$ where $x$ is the element of the group we are considering, and $1$ is the identity element in the group.



        In your case, when your professor says that $|omega| = 3$, what he means is that if you compose the function $omega in S_4$ with itself 3 times, you will get back the identity element of $S_4$. The identity element of $S_4$ is the identity function on ${1,2,3,4}$, i.e. the function:



        $$i = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr)$$



        For instance, although $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$, we see that $omega^2 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 4 & 2 & 1 & 3 end{smallmatrix}bigr)$, and $omega^3 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr) = i$






        share|cite|improve this answer











        $endgroup$


















          2












          $begingroup$

          We first need to know that when talking about $S_4$, we are talking about the set of all bijections from the set ${1,2,3,4}$ to itself.



          So, the elements of the set $S_4$ are functions. An example of a function in $S_4$ is the function $f: {1,2,3,4} to {1,2,3,4}$ given by:
          $$1 mapsto 2, 2mapsto 3, 3mapsto 4, 4 mapsto 1 $$



          However, this is incredibly tedious to write out. Instead, we will use matrices to represent functions in $S_4$. In other words, we will let:



          $$f := bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$$
          where an element in row 1 of the matrix is mapped to the element below it in row 2 of the matrix by $f$.



          Now, when we are talking about order, we can talk about 2 things: the order of a group, and the order of an element of the group.



          The order of a group is the number of elements in that set, but we can also pick out an element from that group (in this case the group is $S_4$ and we're picking out an element from $S_4$) and we can ask what is the order of the element. The order of an element is the smallest positive number $n$ such that $x^n = 1$ where $x$ is the element of the group we are considering, and $1$ is the identity element in the group.



          In your case, when your professor says that $|omega| = 3$, what he means is that if you compose the function $omega in S_4$ with itself 3 times, you will get back the identity element of $S_4$. The identity element of $S_4$ is the identity function on ${1,2,3,4}$, i.e. the function:



          $$i = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr)$$



          For instance, although $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$, we see that $omega^2 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 4 & 2 & 1 & 3 end{smallmatrix}bigr)$, and $omega^3 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr) = i$






          share|cite|improve this answer











          $endgroup$
















            2












            2








            2





            $begingroup$

            We first need to know that when talking about $S_4$, we are talking about the set of all bijections from the set ${1,2,3,4}$ to itself.



            So, the elements of the set $S_4$ are functions. An example of a function in $S_4$ is the function $f: {1,2,3,4} to {1,2,3,4}$ given by:
            $$1 mapsto 2, 2mapsto 3, 3mapsto 4, 4 mapsto 1 $$



            However, this is incredibly tedious to write out. Instead, we will use matrices to represent functions in $S_4$. In other words, we will let:



            $$f := bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$$
            where an element in row 1 of the matrix is mapped to the element below it in row 2 of the matrix by $f$.



            Now, when we are talking about order, we can talk about 2 things: the order of a group, and the order of an element of the group.



            The order of a group is the number of elements in that set, but we can also pick out an element from that group (in this case the group is $S_4$ and we're picking out an element from $S_4$) and we can ask what is the order of the element. The order of an element is the smallest positive number $n$ such that $x^n = 1$ where $x$ is the element of the group we are considering, and $1$ is the identity element in the group.



            In your case, when your professor says that $|omega| = 3$, what he means is that if you compose the function $omega in S_4$ with itself 3 times, you will get back the identity element of $S_4$. The identity element of $S_4$ is the identity function on ${1,2,3,4}$, i.e. the function:



            $$i = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr)$$



            For instance, although $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$, we see that $omega^2 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 4 & 2 & 1 & 3 end{smallmatrix}bigr)$, and $omega^3 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr) = i$






            share|cite|improve this answer











            $endgroup$



            We first need to know that when talking about $S_4$, we are talking about the set of all bijections from the set ${1,2,3,4}$ to itself.



            So, the elements of the set $S_4$ are functions. An example of a function in $S_4$ is the function $f: {1,2,3,4} to {1,2,3,4}$ given by:
            $$1 mapsto 2, 2mapsto 3, 3mapsto 4, 4 mapsto 1 $$



            However, this is incredibly tedious to write out. Instead, we will use matrices to represent functions in $S_4$. In other words, we will let:



            $$f := bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$$
            where an element in row 1 of the matrix is mapped to the element below it in row 2 of the matrix by $f$.



            Now, when we are talking about order, we can talk about 2 things: the order of a group, and the order of an element of the group.



            The order of a group is the number of elements in that set, but we can also pick out an element from that group (in this case the group is $S_4$ and we're picking out an element from $S_4$) and we can ask what is the order of the element. The order of an element is the smallest positive number $n$ such that $x^n = 1$ where $x$ is the element of the group we are considering, and $1$ is the identity element in the group.



            In your case, when your professor says that $|omega| = 3$, what he means is that if you compose the function $omega in S_4$ with itself 3 times, you will get back the identity element of $S_4$. The identity element of $S_4$ is the identity function on ${1,2,3,4}$, i.e. the function:



            $$i = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr)$$



            For instance, although $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$, we see that $omega^2 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 4 & 2 & 1 & 3 end{smallmatrix}bigr)$, and $omega^3 = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 1 & 2 & 3 & 4 end{smallmatrix}bigr) = i$







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited 5 hours ago

























            answered 5 hours ago









            user516079user516079

            363210




            363210























                0












                $begingroup$

                In "cycle notation ", $omega=(134)$ is a $3$-cycle, and $tau=(1234)$ is a $4$-cycle.



                It happens that $n$-cycles always have order $n$.






                share|cite|improve this answer









                $endgroup$


















                  0












                  $begingroup$

                  In "cycle notation ", $omega=(134)$ is a $3$-cycle, and $tau=(1234)$ is a $4$-cycle.



                  It happens that $n$-cycles always have order $n$.






                  share|cite|improve this answer









                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    In "cycle notation ", $omega=(134)$ is a $3$-cycle, and $tau=(1234)$ is a $4$-cycle.



                    It happens that $n$-cycles always have order $n$.






                    share|cite|improve this answer









                    $endgroup$



                    In "cycle notation ", $omega=(134)$ is a $3$-cycle, and $tau=(1234)$ is a $4$-cycle.



                    It happens that $n$-cycles always have order $n$.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered 4 hours ago









                    Chris CusterChris Custer

                    13.9k3827




                    13.9k3827























                        0












                        $begingroup$

                        We may always write a permutation as 1 or more cyclic permutations:



                        For example, for $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$ you may trace the path of element $1$ and see that



                        $$1 mapsto 3 mapsto 4 mapsto 1$$



                        is a cyclic permutation of 3 elements:
                        enter image description here



                        Tracing the remaining element $2$ we see that



                        $$2 mapsto 2$$



                        which is identity of 1-element set, considered as a cyclic permutation, too.



                        Using the notation for cyclic permutations, the first one is



                        $$(1 3 4)$$



                        and the second one is simply $$(2)$$



                        As the result, we may write



                        $$omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)= (1 3 4) (2)$$



                        Now, the first, 3-element cyclic permutation
                        has the order $mathbb 3$ (its "cycle" must turn 3-times $120^o$ to return to its initial position)
                        enter image description here



                        and the the order of the second one is $mathbb 1$ (order of identity). Their least common multiple is $lcm(3, 1)=3$, so



                        $$ord(omega) = 3$$





                        For $tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$ is the situation very simple, because it is a cyclic one :



                        $$1 mapsto 2 mapsto 3 mapsto 4 mapsto 1$$



                        so we may write



                        $$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr) = (1 2 3 4)$$



                        and



                        $$ord(tau) = 4$$






                        share|cite|improve this answer











                        $endgroup$


















                          0












                          $begingroup$

                          We may always write a permutation as 1 or more cyclic permutations:



                          For example, for $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$ you may trace the path of element $1$ and see that



                          $$1 mapsto 3 mapsto 4 mapsto 1$$



                          is a cyclic permutation of 3 elements:
                          enter image description here



                          Tracing the remaining element $2$ we see that



                          $$2 mapsto 2$$



                          which is identity of 1-element set, considered as a cyclic permutation, too.



                          Using the notation for cyclic permutations, the first one is



                          $$(1 3 4)$$



                          and the second one is simply $$(2)$$



                          As the result, we may write



                          $$omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)= (1 3 4) (2)$$



                          Now, the first, 3-element cyclic permutation
                          has the order $mathbb 3$ (its "cycle" must turn 3-times $120^o$ to return to its initial position)
                          enter image description here



                          and the the order of the second one is $mathbb 1$ (order of identity). Their least common multiple is $lcm(3, 1)=3$, so



                          $$ord(omega) = 3$$





                          For $tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$ is the situation very simple, because it is a cyclic one :



                          $$1 mapsto 2 mapsto 3 mapsto 4 mapsto 1$$



                          so we may write



                          $$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr) = (1 2 3 4)$$



                          and



                          $$ord(tau) = 4$$






                          share|cite|improve this answer











                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            We may always write a permutation as 1 or more cyclic permutations:



                            For example, for $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$ you may trace the path of element $1$ and see that



                            $$1 mapsto 3 mapsto 4 mapsto 1$$



                            is a cyclic permutation of 3 elements:
                            enter image description here



                            Tracing the remaining element $2$ we see that



                            $$2 mapsto 2$$



                            which is identity of 1-element set, considered as a cyclic permutation, too.



                            Using the notation for cyclic permutations, the first one is



                            $$(1 3 4)$$



                            and the second one is simply $$(2)$$



                            As the result, we may write



                            $$omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)= (1 3 4) (2)$$



                            Now, the first, 3-element cyclic permutation
                            has the order $mathbb 3$ (its "cycle" must turn 3-times $120^o$ to return to its initial position)
                            enter image description here



                            and the the order of the second one is $mathbb 1$ (order of identity). Their least common multiple is $lcm(3, 1)=3$, so



                            $$ord(omega) = 3$$





                            For $tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$ is the situation very simple, because it is a cyclic one :



                            $$1 mapsto 2 mapsto 3 mapsto 4 mapsto 1$$



                            so we may write



                            $$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr) = (1 2 3 4)$$



                            and



                            $$ord(tau) = 4$$






                            share|cite|improve this answer











                            $endgroup$



                            We may always write a permutation as 1 or more cyclic permutations:



                            For example, for $omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)$ you may trace the path of element $1$ and see that



                            $$1 mapsto 3 mapsto 4 mapsto 1$$



                            is a cyclic permutation of 3 elements:
                            enter image description here



                            Tracing the remaining element $2$ we see that



                            $$2 mapsto 2$$



                            which is identity of 1-element set, considered as a cyclic permutation, too.



                            Using the notation for cyclic permutations, the first one is



                            $$(1 3 4)$$



                            and the second one is simply $$(2)$$



                            As the result, we may write



                            $$omega = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 3 & 2 & 4 & 1 end{smallmatrix}bigr)= (1 3 4) (2)$$



                            Now, the first, 3-element cyclic permutation
                            has the order $mathbb 3$ (its "cycle" must turn 3-times $120^o$ to return to its initial position)
                            enter image description here



                            and the the order of the second one is $mathbb 1$ (order of identity). Their least common multiple is $lcm(3, 1)=3$, so



                            $$ord(omega) = 3$$





                            For $tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr)$ is the situation very simple, because it is a cyclic one :



                            $$1 mapsto 2 mapsto 3 mapsto 4 mapsto 1$$



                            so we may write



                            $$tau = bigl(begin{smallmatrix}1 & 2 & 3 & 4 \ 2 & 3 & 4 & 1 end{smallmatrix}bigr) = (1 2 3 4)$$



                            and



                            $$ord(tau) = 4$$







                            share|cite|improve this answer














                            share|cite|improve this answer



                            share|cite|improve this answer








                            edited 20 mins ago

























                            answered 2 hours ago









                            MarianDMarianD

                            5381511




                            5381511






























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