After looking at how Iwasawa's Lemma proves the simplicity of $PSL_n(q)$ and $PSp_{2n}(q)$ groups, I wondered how powerful this lemma is. As such, I thought about this possible converse.
For reference, Iwasawa's Lemma:
For group $G$,
- $G$ has a primitive group action $\phi:G\rightarrow\text{Sym}(X)$ on set $X$ (transitive, and $\text{Stab}(x)$ is maximal subgroup of $G$)
- $G=G'$ (commutator subgroup)
- There is a soluble normal subgroup $A$ of $\text{Stab}(x)$ such that $G=\langle gAg^{-1}|g\in G\rangle$.
Then $G/\text{ker}(\phi)$ is simple.
A possible converse can be postulated:
For every simple group $G$, $G$ can be expressed as $H/N$, where
- $H$ has a primitive group action $\phi:H\rightarrow\text{Sym}(X)$ on set $X$ (transitive, and $\text{Stab}(x)$ is maximal subgroup of $H$)
- $H=H'$ (commutator subgroup)
- There is a soluble normal subgroup $A$ of $\text{Stab}(x)$ such that $H=\langle hAh^{-1}|h\in H\rangle$.
- $N=\text{ker}(\phi)$
The overarching question is: Which simple groups can possibly satisfy the converse of Iwasawa's Lemma?
You may assume Classification of Finite Simple Groups.
Here's a possible attempt:
As we know the alternating groups $A_n (n\geq5)$ and cyclic groups of prime order $C_p$ are simple, we can try to look at them first. However, the natural action $A_n$ acting on $\{1...n\}$ does not work because the stabilisers are isomorphic to $A_{n-1}$ with no nontrivial normal subgroups for $n\geq 6$.
On examination, it follows that $G$ must act transitively on $X$ too. However, for $G=C_p$, this means that $|X|=p$ otherwise the action is trivial. I'm not sure how to further analyse this case, but it doesn't seem possible for abelian simple groups.
It was observed that $A_6$ can act on left cosets of $S_4$ (with inclusion map $\sigma\mapsto\sigma$ if $\sigma$ is even and $\sigma(5,6)$ otherwise) in $A_6$ via left multiplication. $A_6$ be expressed as $A_6/\{e\}$ with a maximal subgroup isomorphic to $S_4$ (standard twisted $S_4$). $S_4$ trivially has a soluble normal subgroup $A_4$, containing the 3-cycle $(1,2,3)$, whose conjugates generate $A_6$. This doesn't seem to generalise easily to the other altermating groups.