theorem Th14:
  for i,j being Integer st i,j are_coprime holds
  i <> j or i = j = 1 or i = j = -1
  proof
    let i,j be Integer;
A1: i gcd i = |.i.| by INT_5:12;
    |.i.| = i or |.i.| = -i by ABSVALUE:1;
    hence thesis by A1;
  end;
