# Homework Help: How many elements of order 2 are contained in S_4?

1. Jun 27, 2011

### Samuelb88

1. The problem statement, all variables and given/known data
How many elements of order 2 does the symmetric group $S_4$ contain?

2. Relevant equations

3. The attempt at a solution
I know that transpositions have order two. Moreover, any k-cycle has order k. Thus there are six elements with order two contained in $S_4$?

Is this all? Seems too simple.

2. Jun 27, 2011

### ehild

How many elements are there in S4 and what are they?

ehild

3. Jun 27, 2011

### Samuelb88

ehild,

There are 4! = 24 elements in $S_4$. The elements are all such permutations of {1,2,3,4}.

Here is how I see the problem:

Let $\sigma \in S_4$. Then either:
1. $\sigma = (a_1 a_2)$ and $(a_1 a_2)^2 = e$ $\Rightarrow$ order 2.
2. $\sigma = (a_1 a_2 a_3$ and $(a_1 a_2 a_3)^3 =e$ $\Rightarrow$ order 3.
3. $\sigma = (a_1 a_2 a_3 a_4)$ and $(a_1 a_2 a_3 a_4)^4 = e$ $\Rightarrow$ order 4.

Thus there are 6 permutations of order 2 in $S_4$. Is this not correct?

Last edited: Jun 27, 2011
4. Jun 27, 2011

### ehild

Oh, you mean the permutation group? I thought it was the point group called S4 - sorry.

ehild

5. Jun 27, 2011

### I like Serena

No, this is not correct.
Did you find how many of order 3 and 4 there are?
Do they add up to 24?

6. Jun 27, 2011

### ehild

7. Jun 27, 2011

well, I got an idea but am not sure if I'm right cuz I haven't had abstract algebra yet. well, what you're claiming is that in a cyclic group G an element in G is a transposition if and only if It is of order 2. well, It's obvious that any transposition element in G is of order 2 but can we say that any element of order 2 is a transposition?
if yes, then your question would become that in how many ways we can permute two letters from n letters keeping the others the same position they are. That would be an easy problem in combinatorics and discrete math.

8. Jun 27, 2011

### micromass

Hi Samuelb88!

You are certainly correct that there are 6 transpositions in S4, i.e. there are 6 elements of the form (a b). These are

$$(1~2),(1~3),(1~4),(2~3),(2~4),(3~4)$$

However, these are not the only elements of order 2!! For example

$$(1~2)(3~4)$$

is also of order 2, so you got to count this one too!!

9. Jun 27, 2011

### Samuelb88

Oh right! So that means there are six transpositions and three disjoint transpositions in [itex]S_4[\itex].

10. Jun 27, 2011

### micromass

Yes, that sounds right!

11. Jun 27, 2011

Thanks guys!