Proving a property when elements of a group commute

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Discussion Overview

The discussion revolves around the necessity of proving that elements of a group commute when the group is known to be commutative. Participants explore whether the commutativity of individual elements implies the commutativity of their powers.

Discussion Character

  • Debate/contested

Main Points Raised

  • Some participants assert that if a group is commutative, then the equality ##a^n b^m = b^m a^n## should be automatically implied without separate proof.
  • Others argue that while ##a^n## and ##b^m## are elements of the group, their equality may not hold in non-Abelian groups, suggesting that a separate proof is necessary.
  • A participant acknowledges a misunderstanding regarding the nature of the group, realizing that the group is not actually abelian.
  • One participant suggests that the proof appears trivial if the group is known to be commutative.
  • Another participant provides a proof outline involving the manipulation of powers of the elements.

Areas of Agreement / Disagreement

Participants express differing views on whether a separate proof is necessary for the commutativity of powers in a commutative group. The discussion remains unresolved regarding the necessity of the proof in the context of non-Abelian groups.

Contextual Notes

Participants discuss the implications of group properties and the definitions of commutativity, highlighting the potential for misunderstanding when the nature of the group is not clearly defined.

Mr Davis 97
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By commutative, we know that ##ab = ba## for all a,b in G. Thus, why do we need to prove separately that ##a^n b^m = b^ma^n##? Isn't it the case that ##a^n## and ##b^m## are in fact elements of the group? So shouldn't the fact that they commute automatically be implied?
 
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Mr Davis 97 said:
Isn't it the case that anana^n and bmbmb^m are in fact elements of the group?
Yes but each of ##a^n## and ##b^m## might equal another element of the group and they need not commute if the group is non-Abelian.
 
blue_leaf77 said:
Yes but each of ##a^n## and ##b^m## might equal another element of the group and they need not commute if the group is non-Abelian.
Oh, I see. I neglected to see that the group is not actually abelian
 
The proof you want seems trivial.
 
Mr Davis 97 said:
By commutative, we know that ##ab = ba## for all a,b in G. Thus, why do we need to prove separately that ##a^n b^m = b^ma^n##? Isn't it the case that ##a^n## and ##b^m## are in fact elements of the group? So shouldn't the fact that they commute automatically be implied?
Why do you believe you need to prove it separately?
 
Mr Davis 97 said:
By commutative, we know that ##ab = ba## for all a,b in G. Thus, why do we need to prove separately that ##a^n b^m = b^ma^n##? Isn't it the case that ##a^n## and ##b^m## are in fact elements of the group? So shouldn't the fact that they commute automatically be implied?

If you know that the group is commutative then there is nothing to prove. ##a^nb^m = b^ma^n## by definition. If you do not know that the group is commutative then you need a proof.
 
Last edited:
Proof: a^nb^m=a^{n-1}bab^{m-1}, etc.
 

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