# Homework Help: Permutation groups

1. Dec 14, 2008

### VeeEight

I have two questions, they aren't homework questions but I figured this would be the best place to post them (they are for studying for my exam).

1. The problem statement, all variables and given/known data

How many elements of S_6 have order 4? Do any elements have order greater than 7?

2. Relevant equations

S_6 is the permutation group on {1, 2,..,6}. The order, n, of an element here is a permutation f such that f^n = 1

3. The attempt at a solution

I figured that it wouldn't be wise to check all the elements since there are so many of them. I know the order of an element has to divide the order of the group. The order of S_6 is 6!, which seven does not divide so no elements have order 7 (but there must be an element of order 6 since 6 divides 6!). But there must be an element of order 9 since 9|6!. I am not sure if this logic is correct and how to actually find how many elements there are of each order in general.

1. The problem statement, all variables and given/known data

Prove that every odd permutation in S_n has even order

2. Relevant equations

An odd permutation is a permutation that can be written as a product of an odd number of transpositions (2-cycles). If a permutation is odd then it's sgn is -1

3. The attempt at a solution

My instinct is to incorporte the sgn function into the proof but I am unsure of how to use it.

2. Dec 15, 2008

### CompuChip

You are right about the first argument: since 7 does not divide 6! there are no elements of order 7. However, it is not true that for every divisor of 6! there must be an element with that order. For example, the group A4 (odd permutations in S4) does not have any element of order 6. In general, it's hard to say something about the number of subgroups that do exist; usually the Sylow theorems are useful in such cases.

What order does the identity have? What happens to the sign if you compose n odd permutations?

3. Dec 15, 2008

### Dick

As far as counting elements of order 4, can you relate the order of an element to the cycle structure of the permutation in the case where it has order 4? Actually, that can probably help you with the 7 or greater question as well.

Last edited: Dec 15, 2008
4. Dec 15, 2008

### VeeEight

thanks! I believe I have the answer