
theorem
  for f being Arithmetic_Progression st
    for i being Nat holds f.i is Prime holds
      difference f is Integer
  proof
    let f be Arithmetic_Progression;
    assume for i being Nat holds f.i is Prime; then
A2: f.0 is Prime & f.1 is Prime;
    difference f = f.1 - f.0 by NUMBER06:def 5;
    hence thesis by A2;
  end;
