Every subgroup of index 2 is normal?

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Homework Help Overview

The discussion revolves around the proof that every subgroup H of a group G with index 2 is normal. The subject area is group theory, specifically focusing on subgroup properties and cosets.

Discussion Character

  • Exploratory, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the requirement to show that gN=Ng for all g and question the nature of the cosets of N. There is an exploration of how elements from H and its complement relate to the cosets.

Discussion Status

Some participants have provided guidance on the relationship between elements of H and its complement, noting that both cosets must be equal to H and its complement. There is an ongoing exploration of the implications of these relationships.

Contextual Notes

Participants are working under the constraints of proving a property of subgroups without assuming prior knowledge of specific group structures or additional theorems.

Oster
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I have to prove that every subgroup H of a group G with index(number of distinct cosets of the subgroup) 2 is normal.

I don't know how to start :'(

Please help.
 
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Hi Oster! :smile:

You must prove that gN=Ng for all g. But what exactly are the cosets of N??
 
If the element g comes from H then both gH and Hg are equal to H.
If g comes from H complement then i know it must represent the "other" coset. Since cosets partition G, i know both these cosets must be equal to H complement.
More or less correct?
 
Yes, that's good!
 

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