During my Algebra class we were given this exercise to solve at home, but I couldn't find any solution and I also did not really get the one our teacher gave us. So, the text was:
Given G = S4 = Sym({1, 2, 3, 4}). Then:
- Is there a subgroup of G of order 5? Motivate your answer.
- Give and example of two subgroups H1 and H2 so that |H1| = |H2| and H1 $\ncong$ H2 (H1 not isomorphic to H2)
Part 1 of the question is fairly simple in fact thanks to Lagrange Theorem on subgroups, but the problems arise with Part 2. The given solution is:
Let's consider as an example:
- H1 = { id, (1,2), (3,4), (1,2)(3,4) }
- H2 = { id, (1,2)(3,4), (1,3)(2,4), (1,4)(2,3) }
In fact, H1 elements are all of order 2 (apart from id = identity ) while (1,2,3,4) $\in$ H2 has order 4. Thus H1 $\ncong$ H2 ( H1 not isomorphic with H2 )
I've tried my best, but given the fact that our teacher sometimes is very messy I still can not figure out why this is a valid proof, nor the steps needed to find it. This is my first time posting, so sorry in advance for any mistake. Thank you for your time!
(1,2)(3,4) is a different permutation from (1,2,3,4), and the correct group for $H_2$ is {id,(1,2,3,4),(1,3)(2,4),(1,4,3,2)}.