12. A normal subgroup H of a group G is said to be a direct factor (direct summand if
G is additive abelian) if there exists a (normal) subgroup K of G such that
G = HX K.
(a) If H is a direct factor of K and K is a direct factor of G, then H is normal
(b) If H is a direct factor of G, then every homomorphism H —>G may be
extended to an endomorphism G -> G. However, a monomorphism H -> G need
not be extendible to an automorphism G —> G.