Is There a Normal Subgroup K in Groups G and H with Index (G:K) ≤ n!?

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Discussion Overview

The discussion revolves around the existence of a normal subgroup K in groups G and H, given that the index (G:H) is equal to n. Participants explore the implications of this relationship, including examples and theoretical reasoning.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant states that if (G:H)=n, then there exists a normal subgroup K of G such that (G:K)≤n!, providing an example with G=A5 and H=A4.
  • Another participant suggests that this result can be understood through the action of G on the cosets of H, leading to a homomorphism of G into the group of permutations of these cosets, and mentions the kernel of this homomorphism.
  • A later reply notes that (G:K) can be expressed as dividing n!, which may be more useful in certain contexts.

Areas of Agreement / Disagreement

Participants present differing perspectives on the implications and utility of the existence of the normal subgroup K, with no consensus reached on the best approach or interpretation of the results.

Contextual Notes

Some assumptions about the nature of groups G and H (finite or infinite) and the specific properties of the normal subgroup K are not fully explored, leaving open questions about the general applicability of the claims made.

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G,H be groups(finite or infinite)
Prove that if (G:H)=n, then there exist some normal subgroup K of G (G:K)≤n!
example) let G=A5, H=A4 then (G:H)=5, then K={id} exists, (G:K)≤5!
 
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This is a standard result. If you think how elements of G can act on the left (or right) cosets of H you should come up with a homomorphism of G into the group of permutations of the cosets. Then think about the kernel of the homomorphism.
 
Actually you have (G:K)|n! which is sometimes more useful.
 
Thanks a lot Martin. I understand it.
 

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