# Symmetric group (direct product and decomposition)

1. Dec 11, 2008

### Symmetryholic

I am looking for a mechanism to find a decomposition of symmetric groups. For finitely generated abelian group G, there is a mechanism to decompose G such that G is isomorphic to a direct sum of cyclic groups.

For symmetric groups, it seems a bit complex for me to find it.

For example,

(1) Is it possible for $$S_{n}$$ to be decomposed as a direct product of groups?

(2) Is there any mechanism to find an isomorphic group of a direct product of symmetric groups? Let's say,

$$G = S_{n-2} \times S_{n-1} \times S_{n}$$

Any isomorphic group of the above G?

2. Dec 11, 2008

### rodigee

(1) No, you can't decompose the symmetric group into the direct product of two (or more!) of it's proper subgroups.

(2) Sorry, but I don't understand the question.

3. Dec 11, 2008

### Symmetryholic

If we cannot decompose symmetric groups, question (2) might reduce to find isomorphic groups of $$S_{n}$$.

Is there any known group, let's say P, isomorphic to $$S_{n}$$? If so, by Caley's theorem, any group G is isomorphic to the subgroup of P. Is that right?

4. Dec 11, 2008

I'm not understanding you.

What do you mean by "isomorphic groups of Sn"? Groups isomorphic to Sn? You can take any set you like of size n! and define a group operation on it to make it isomorphic to Sn.
Yes, put P = Sn. Or do you mean something different?
No, just pick a group G larger than P. Cayley's theorem states that any finite group G is isomorphic to a subgroup of Sn, where n = |G| (it depends on G!).

5. Dec 11, 2008

### Symmetryholic

Like $$A_{n+2}$$, is there any known group that has a subgroup isomorphic to $$S_{n}$$? $$A_{n+2}$$ is the only group (different from $$S_n$$ ) I have found so far.

Is there any dihedral group(or variants) of order m > n that has a subgroup isomorphic to $$S_{n}$$?

Last edited by a moderator: Apr 24, 2017
6. Dec 11, 2008

### rodigee

Right, $$A_{2n}$$ is trivially isomorphic to $$S_n$$. Even more trivially, $$S_{n+k}$$ has plenty of subgroups isomorphic to $$S_n$$ for all $$k\geq 1$$. Continuing, so does $$S_n \times G$$ for any group $$G$$... etc. You can construct as many as you like which is why I (and probably adriank as well) am confused by this "any known group" phrase.

I don't know then answer to your Dihedral group question off the top of my head. I'll think about it. Assuming you haven't already, see if you can figure out why the symmetric group can't be decomposed :tongue:

7. Dec 11, 2008

Not true: |A2n| = (2n)!/2, while |Sn| = n!. However, An+2 has a subgroup isomorphic to Sn, at least according to the post he linked.

As for the dihedral group question: I write Dn to mean the dihedral group of order 2n. Any subgroup of Dn is either cyclic or isomorphic to Dm, where m divides n.

Every dihedral group has its trivial subgroup isomorphic to S1. Every dihedral group also has a subgroup isomorphic to S2, which is a cyclic group of order 2. For S3, note that S3 is isomorphic to D3, so the dihedral groups that have a subgroup isomorphic to S3 are exactly the ones of the form D3n. Thereafter, no symmetric group is isomorphic to a dihedral group, so there are no more.

8. Dec 12, 2008

### rodigee

I meant to say " $$A_{2n}$$ has a subgroup that is trivially isomorphic to $$S_n$$"

9. Dec 12, 2008

Alright, I guess that's true. :)

10. Dec 12, 2008

### mathwonk

the analog for non abelian groups, of the direct decomposition of abelian ones, is the filtration by subnormal towers, used in galois theory. see the jordan-holder theorem. or the krull schmidt theorem.

11. Dec 13, 2008

I was going to list some composition series for the symmetric groups, but I had a feeling that those wouldn't be what he was looking for. I may as well do it anyway...

Here are composition series with the composition factors for the symmetric groups.
$$S_1 = \{\varepsilon\}$$
$$S_2 \rhd \{\varepsilon\}$$; factors $$C_2$$
$$S_3 \rhd A_3 \rhd \{\varepsilon\}$$; factors $$C_2, C_3$$
$$S_4 \rhd A_4 \rhd K \rhd H \rhd \{\varepsilon\}$$, where $$K = \{\varepsilon, (12)(34), (13)(24), (14)(23)\}$$ and $$H = \{\varepsilon, (12)(34)\}$$; factors $$C_2, C_3, C_2, C_2$$
$$S_n \rhd A_n \rhd \{\varepsilon\}, n \ge 5$$; factors $$C_2, A_n$$

12. Dec 13, 2008

### Symmetryholic

Thanks for all replies.

I have some further questions.

(1) Is it possible for $$S_{n}$$ (n>=5) to be a product of its proper subgroups such that $$S_{n} = HK$$. H and K are proper subgroups of $$S_{n}$$.
(2) Is it possible for $$S_{n}$$ to be a finite product of its proper subgroups such that $$S_{n} = H_{1}H_{2}..H_{k}$$, where $$H_{1}H_{2}..H_{k}$$ are proper subgroups of $$S_{n}$$, where n >=5.

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Lemma 1. $$S_{n}$$ is generated by a=(1 2) and b=(1 2 3 ... n).(http://mathforum.org/library/drmath/view/51685.html")
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My assumption is
If $$b \in A_{n}$$ for n>=5, it is possible for (1). (Is this correct?)
(It's because $$a \in S_{2}$$ and $$b \in A_{n}$$, and both $$S_{2}, A_{n}$$ are proper subgroups of $$S_{n}$$ ).
If $$b \notin A_{n}$$, I have no clue.

Last edited by a moderator: Apr 24, 2017
13. Dec 13, 2008

It's possible for any n > 2; take H = {e, (12)} and K = An. In that case, HK = An ∪ (12)An = Sn; this is the union of the even permutations of Sn and the odd permutations of Sn.

14. Dec 13, 2008

### Symmetryholic

Is it possible for $$A_{n}$$ to be a product of its proper subgroups as well such that $$A_{n} = H_{1}H_{2}..H_{k}$$, where $$H_{1}H_{2}..H_{k}$$ are proper subgroups of $$A_{n}$$, where n >=5 ?

This problem is hard for me as well because I could not find any pattern on proper subgroups of $$A_{n}$$.