The problem statement, all variables and given/known data Recall that a subgroup N of a group G is called characteristic if f(N) = N for all automorphisms f of G. If N is a characteristic subgroup of G, show that N is a normal subgroup of G. The attempt at a solution I must show that if g is in G, then gN = Ng. Let n be in N. Since N is characteristic, there are automorphisms f and f' of G and elements a and a' of G such that n = f(a) = f'(a'). There are also elements b and b' in G such that g = f(b) = f'(b') so that gn = f(b)f(a) = f(ba) = f'(b')f(a) = f(b)f'(a') = f'(b')f'(a') = f'(b'a'). This is all I can think of and I don't see how this allows me to prove that N is normal. Am I missing something?