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Counterexample

Is this true?

H ⊴ K and K ⊴ G imply H ⊴ G

No — it is false.

The counterexample

⟨(12)(34)⟩ ⊴ V4V_{4} ⊴ A4A_{4}

Why it fails

Conjugating (12)(34) by the 3-cycle (123) gives (13)(24) ∉ ⟨(12)(34)⟩.

Go deeper· Explained in full in Normal subgroups, quotients and the isomorphism theorems· Asked in CSIR NET June 2025· From Groups › Normal subgroups, quotients, isomorphism theorems

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