Here is the statement on my exercise sheet :
Let $G$ be a finite group, $p$ the smallest prime factor of $\vert G \vert$ and $H$ a subgroup of $G$ whose index is $p$. Then $H$ is a normal subgroup of $G$.
I already know how to prove this powerful statement. But I searched on the literature to find more about that nomination but I could not find anything relevant. It often brought me back to some Frobenius results but nothing more...
If someone has references about it or has already heard about the name of that lemma, it would be great to share !
Thanks in advance !