# Math Help - Isomorphism

1. ## Isomorphism

Find the minimum of all positive integers m such that every sigma in S_9 (Sigma in A_9) satisfies sigma^ n = 1.

Find 4 different subgroups of S_4, isomorphic to S_3, and nine isomorphic to S_2. ( I dont need all the subgroups just how to find them isomorphic)

Let G be a subgroup of S_5. Prove that if G contains a 5-Cycle and a 2-Cycle, then G = S_5.

2. Originally Posted by Juancd08
Find the minimum of all positive integers $m$ such that every $\sigma \in S_9$ ( $\sigma \in A_9$) satisfies $\sigma^ n = 1$.
Hint: for $\sigma = (a_1 a_2 \ldots a_i)$ a permutation of $i$ objects then $\sigma$ has order precisely $i$. Prove this, then apply it to your questions.

Originally Posted by Juancd08
Find 4 different subgroups of $S_4$, isomorphic to $S_3$, and nine isomorphic to $S_2$.
To find the subgroups isomorphic to $S_2$ note that $S_2 \equiv C_2 (=\mathbb{Z}_2)$, the cyclic group of 2 elements. So, how many elements in this group have order 2?...

For $S_3$, your groups are just the permutations of 3 elements, $\{a_1, a_2, a_3\} \subset \{1,2,3,4\}$. This is because $S_3$ is merely the group of all possible permutations of 3 objects, by it's "rawest" definition. If you take all possible permutations of 3 objects you will get a group isomorphic to $S_3$.

Originally Posted by Juancd08
Let G be a subgroup of S_5. Prove that if G contains a 5-Cycle and a 2-Cycle, then G = S_5.
I suspect someone here can provide you with a hint towards a much neater proof than I have. Sorry. However, I would advise you to try this for smaller values of $n$ to see how it works. Note that $S_n$ is generated by a transposition and an $n$-cycle for all $n > 2$