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Recall that a group $ G $ is called almost simple if there exists a (non-abelian) simple group $ S $ such that $ S \leq G \leq {\rm Aut}(S) $.

If $ G $ is almost simple then $ {\rm Soc}(G) $, the socle of $ G $, is (non-abelian) simple.

See The socle of an almost simple group or Is the Socle of an almost simple group a simple group?

What about the converse? In other words, if ${\rm Soc}(G) $ is (non-abelian) simple then must $ G $ be almost simple?

HeroZhang001
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1 Answers1

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Yes.

Say $N \trianglelefteq G$ with $N$ non-abelian simple and $\operatorname{soc}(G) = N$. If $C_G(N) \neq 1$, then $C_G(N)$ contains a minimal normal subgroup of $G$, which must be $N$. But this is impossible since $N$ is non-abelian, so $C_G(N) = 1$.

Thus the conjugation action of $G$ on $N$ defines an injective homomorphism $G \rightarrow \operatorname{Aut}(N)$. So $G$ embeds into $\operatorname{Aut}(N)$ with $N \leq G \leq \operatorname{Aut}(N)$.

spin
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  • @runway44: I don't remember what was the argument I had in mind anymore, but I edited to something that makes more sense to me. – spin Mar 18 '23 at 01:16