Theorem SG1
If G is a group of transformations of a set S, and F is a subset of S, then
S(F,G) is a subgroup of G.
Proof
(1) Since e, the identity of G, has e(F) = F, eεS(F,G).
(2) For gεS(F,G), g(F) = F, so g^{1}(F) = F
and hence g^{1}εS(F,G).
(3) For g and hεS(F,G), g(F) = F and h(F) = F,
so that we have
goh(F) = g(h(F)) = g(F) = F.
Then gohεS(F,G).
Together, these show that we have a subgroup of G.

