Verifying Noncyclic Abelian Subgroup of S4

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Discussion Overview

The discussion revolves around verifying that the set {(1), (1 2), (3 4), (1 2)(3 4)} forms an Abelian, noncyclic subgroup of S4. Participants explore the definitions and criteria for determining whether a group is cyclic or noncyclic, as well as the implications of these properties.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that a cyclic group is generated by a single element.
  • Another participant suggests that to show a group is noncyclic, one must demonstrate that there exists a member that cannot be expressed as a power of any other member.
  • A participant questions whether the exponent n must be non-negative, proposing that negative integers should also be considered to account for inverses.
  • One participant asserts that the group is noncyclic by stating that for any element g in the group, g squared equals the identity, and since the order of the group is 4, no single element can generate the group.

Areas of Agreement / Disagreement

Participants generally agree on the criteria for a group being cyclic or noncyclic, but there are differing views on the necessity of non-negative integers for exponents in group operations. The discussion remains unresolved regarding the implications of this point.

Contextual Notes

There is a lack of consensus on the treatment of negative integers in the context of group operations, which may affect the understanding of cyclicity.

Gear300
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The problem is to verify that {(1), (1 2), (3 4), (1 2)(3 4)} is an Abelian, noncyclic subgroup of S4.

I was able to show that it is Abelian through pairing the permutations, but my mind stopped at the noncyclic part. When showing that a group is cyclic or noncyclic, what exactly do I have to show?
 
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A cyclic group is generated by a single element.
 
Therefore, anyone of those elements should be able to generate the others, right?
 
Yes, if it were cyclic. In order for a group to be cyclic then there must exist a member a so that for all members b, there exists a non-negative integer n so that an=b.

In order to show a group is cyclic, one must find such a member a. To show it is non-cyclic, one must show that there is a member b which cannot be the power of any other member (it is obviously the 1st power of itself).

I'd look at (1 2)(3 4) and see if one can show whether it is a power of any of the others.

--Elucidus
 
Doesn't seem as though (1 2)(3 4) is a power of any of the other elements.
Does n have to be non-negative (in order for it to be a group, shouldn't n also be inclusive of negative integers - to identify the inverses)?
 
The group G that you've presented is certainly noncyclic. Here is a proof: For any element g in G, g2=1. However, the order of the group is 4, and so no single element can generate the group. Thus the group is not cyclic.

Hope that helps!
 
aziz113 said:
The group G that you've presented is certainly noncyclic. Here is a proof: For any element g in G, g2=1. However, the order of the group is 4, and so no single element can generate the group. Thus the group is not cyclic.

Hope that helps!

Thanks for the help.
 

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