Here is the question:
a) Show that if $p$ is a prime number and $P$ is a $p$-subgroup of a finite group $G$, then $[G:P]=[N_G(P):P]$(mod p), where $N_G(P)$ denotes the normalizer of $P$ in $G$.
b) Assume that $H$ is a subgroup of a finite group $G$, and that $P$ is a Sylow $p$-group of $H$. Show that if $N_G(P) \subset H$ then $P$ is a Sylow p-subgroup of $G$.
I have forgotten too much group theory to make much progress on this. I try to look at the action of $P$ on the set of cosets, $G/P$, but I must be missing something because not much connects to anything.
Any help would be very much appreciated.
Best Answer
You can use the result of $1$ to see that $P$ is a $p$-sylow subgroup of finite group $G$. Let $|G|=p^{\alpha}m,(p,m)=1$.
$H\leq G$ so $|H|=p^{s}n,~ s\leq\alpha,~~n\mid m$.
$P$ is a sylow-$p$ subgroup of $H$ so $|P|=p^s$.
$N_G(P)\subseteq H$ so $|N_G(P)|=p^sk,~~k\mid n$.
For all above $m,~n$ and $k$ are free of $p$ that is $(p,m)=1,~(p,n)=1,~(p,k)=1$.
Now apply the congruent relation resulted by $1$ for what we have:
$$[G:P]\equiv [N_G(P):P]~~~(\text{mod}~~ p)$$ or since $G$ is finite: $$|G|/|P|\equiv |N_G(P)|/|P|~~~(\text{mod}~~ p)$$ or $$p^{\alpha}m/p^s\equiv p^s.k/p^s~~~(\text{mod}~~ p)$$ or $$p^{\alpha-s}m\equiv k~~~(\text{mod}~~ p)$$
The latter makes a contradiction unless $\alpha=s$ and $m=k$. This is what you wanted in $2$.