Let h map a subspace A
continuously into the completely regular space S so that A and h[A] are completely
separated in S, and let Q be the quotient space of S gotten by identifying p with h(p)
for all p in A. If there exists a simultaneous extension from C(A) into C(S), then
there exists an isomorphism of C(S) onto itself, taking C(Q) onto C(S∥A), which is
the identity on C(S∥h[A]) (whence C(Q) is complemented in C(S)). The
converse holds providing A and h[A] are normally embedded in S and h is a
homeomorphism.