Re: subgroups of S_n



(1) Consider the subgroup H_i = {s in S_n : s(i) = i}
for some 1 =< i =< n.

Why does H have order (n-1)! Is it isomorphic to
S_{n-1} ?

Orbit-Stabilizer, yes.

(2) Does every subgroup of index n in S_n arise in
this way?

That is, is every subgroup of index n in S_n equal to
H_i for some i ?

Not for n=6, but for other n yes.
.