let G be Group; :: thesis: for f being Element of Aut G holds f " is Element of Aut G
let f be Element of Aut G; :: thesis: f " is Element of Aut G
reconsider f = f as Homomorphism of G,G by Def1;
reconsider A = f " as Homomorphism of G,G by Th5;
f is bijective by Th4;
then A is bijective by GROUP_6:63;
hence f " is Element of Aut G by Def1; :: thesis: verum