Normal Cyclic Subgroup in A_4: Proving Normality and Identifying Elements

Click For Summary

Homework Help Overview

The discussion revolves around the normality of the cyclic subgroup { (1), (123), (132) } within the alternating group A_{4}, which consists of even permutations of four elements. Participants are exploring methods to determine if this subgroup is normal by checking the condition gH = Hg for all g in A_{4} and discussing the identification of elements within A_{4}.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are considering the tedious nature of checking the normality condition directly and are questioning if there are more efficient methods. There is also a discussion about visualizing A_{4} and identifying its elements, with references to its association with the rotations of a regular tetrahedron.

Discussion Status

The conversation is ongoing, with some participants suggesting alternative approaches to proving normality, such as examining specific elements of A_{4} and their interactions with the subgroup. There is an acknowledgment that brute force checking is not the only method available.

Contextual Notes

Participants are grappling with the definitions and properties of even permutations and the implications of cycle structures in relation to normality. The discussion reflects a mix of theoretical exploration and practical concerns about subgroup identification.

polarbears
Messages
23
Reaction score
0

Homework Statement



Is the Cyclic Subgroup { (1), (123), (132)} normal in A_{4} (alternating group of 4)

Homework Equations





The Attempt at a Solution



So I believe if I just check if gH=Hg for all g in A_4 that would be suffice to show that it is a normal subgroup, but that seems really tedious. Is there a easier way?

Also how can I figure out what the elements of A_4 are? I know its the even permutations but is there a way to quickly identity which ones it is? How do I visualize it?
 
Physics news on Phys.org
Visually speaking, A_4 is the group associated with rotations of the regular tetrahedron, if that helps.

One can use the Sylow theorems to prove normality sometimes, but in this case it doesn't help.
 
So only way is by brute force?
 
polarbears said:
So only way is by brute force?

No, it's not the only way. In this case, I would try and guess an element g of A4 such that gHg^(-1) is NOT equal to H, where H is your subgroup. It's not hard. You can recognize whether a permutation is even just by looking at it's cycle structure. Cycles with an odd number of elements are even permutations and cycles with an even number of elements are odd permutations.
 

Similar threads

  • · Replies 15 ·
Replies
15
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K