Are Distinct Left Cosets and Right Cosets in a Group Related?

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Discussion Overview

The discussion revolves around the relationship between distinct left cosets and right cosets in a group, particularly focusing on whether the distinctness of left cosets implies the distinctness of corresponding right cosets. The scope includes theoretical exploration of group theory concepts.

Discussion Character

  • Exploratory, Debate/contested, Conceptual clarification

Main Points Raised

  • One participant suggests that if aH and bH are distinct left cosets, then Ha and Hb must also be distinct right cosets, based on the definitions of cosets.
  • Another participant challenges this by stating that the previous statement is merely a definition and questions whether there exists a counter-example where distinct left cosets do not lead to distinct right cosets.
  • A further contribution notes that when H is a normal subgroup, the statement holds true, but raises concerns about its validity when H is not normal, suggesting that counterexamples should be explored in such cases.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between left and right cosets, with some questioning the validity of the initial claim and others agreeing that the situation changes when considering normal subgroups. The discussion remains unresolved regarding the existence of counterexamples.

Contextual Notes

Participants acknowledge the dependence on whether H is a normal subgroup, which may influence the validity of the claims made. There is also uncertainty about the existence of counterexamples that could challenge the initial assertion.

ivyhawk
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G group, H subgroup of G.
Suppose aH and bH are distinct leftcosets then Ha and Hb must be distinct right cosets?


My humble thoughts:
the left coset aH consists of a times everything in H;
Ha consists of everything in H times a.
Then this argument above is true?
 
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That's not an argument, it's just the definition of the cosets. :smile:
 
Fredrik said:
That's not an argument, it's just the definition of the cosets. :smile:

My doubt is whether there is a counter-example such that when aH and bH are distinct left cosets, Ha and Hb are not distinct right cosets, because this statement looks suspicious.
 
When H is a normal subgroup, it's very easy to prove that the statement is true, but I don't see why it must be true when H isn't a normal subgroup. So if you're looking for counterexamples, start by thinking of examples of subgroups that aren't normal.
 

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