normality of subgroups of prime index
Proposition.
If is a subgroup of a finite group
of index , where is the smallest prime dividing the order of , then is normal in .
Proof.
Suppose with finite and , where is the smallest prime divisor of , let act on the set of left cosets
of in by left , and let be the http://planetmath.org/node/3820homomorphism
induced by this action. Now, if , then for each , and in particular, , whence . Thus is a normal subgroup
of (being contained in and normal in ). By the First Isomorphism Theorem
, is isomorphic to a subgroup of , and consequently must http://planetmath.org/node/923divide ; moreover, any divisor of must also , and because is the smallest divisor of different from , the only possibilities are or . But , which , and consequently , so that , from which it follows that is normal in .∎