theorem Th6:
  for f being Element of Aut G holds f" is Element of Aut G
proof
  let f be Element of Aut G;
  reconsider 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 thesis by Def1;
end;
