theorem Th18:
  for c being Complex
  for f being complex-valued FinSequence holds (f(/)c)^2 = (f^2) (/) (c^2)
  proof
    let c be Complex;
    let f be complex-valued FinSequence;
    thus (f(/)c)^2 = (1/c)^2 (#) (f^2) by Th17
    .= (1*1)/(c*c) (#) (f^2) by XCMPLX_1:76
    .= (f^2) (/) (c^2);
  end;
