Where does the abelian part come in?

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Homework Help Overview

The discussion revolves around proving that the set H of all solutions satisfying the equation x^n = e forms a subgroup of an abelian group G with identity e. Participants are exploring the implications of the group being abelian and questioning the necessity of this condition in the proof.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants attempt to demonstrate that the identity element and inverses are included in H, and they explore the closure property of the set H under the group operation. Questions arise regarding the role of the abelian property in the proof, particularly in relation to the expression (ab)^n.

Discussion Status

There is an ongoing exploration of the relationship between the abelian property of group G and the subgroup H. Some participants express confusion about why G must be abelian, while others reference previous responses that address this concern. The discussion is productive, with participants engaging in practice problems to reinforce their understanding.

Contextual Notes

Participants note that if G is not abelian, the equality (ab)^n = a^n b^n may not hold, which is a critical point in the discussion of the subgroup properties.

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Homework Statement


Prove the set H of all solutions satisfying the equation x^n = e form a subgroup of an abelian group G with identity e.


Homework Equations





The Attempt at a Solution



e^n = e so e belongs to H.

x^n = e => x^(-n)= e => (x^(-1))^n= e so for any x belonging to H, x inverse belongs to H.

if a^n=e and b^n= e => (ab)^n=e so ab belongs to H.

where does the abelian part come in?
 
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if a^n=e and b^n= e => (ab)^n=e so ab belongs to H.

where does the abelian part come in?

(ab)^n = (ab)(ab)(ab)...(ab)(ab)(ab) =(<--here) (aa...aaa)(bb...bb) = a^nb^n = ee = e

note that more is true, the set of all elements of finite order of an abelian group form a subgroup. the proof is the same as this except showing closure is slightly different, you could do it for practice if u wanted
 
Last edited:
i'm going to do the practice problem just to make sure I've got it. if a^n=e and b^s=e then (a^n * b^s)=e. then by the group being abelian (ab)^s * a^(n-s)=e. we can assume n>s without loss of generality. therefore (ab)^s = a^s. if we take both sides to the power (n/s) then we have (ab)^n = e. thus ab belongs to G.
 
by the way i still don't understand why G must be abelian in the first problem.
 
rsa58 said:
by the way i still don't understand why G must be abelian in the first problem.
That was answered in ircdan's first response. If G is not abelian, then (ab)n is NOT, in general, anbn so you cannot use the fact that an= bn= e.
 

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