theorem Th23:
  b > 1 iff b! > 1
  proof
    thus b > 1 implies b! > 1
    proof
      assume b > 1; then
      b >= 1+1 by NAT_1:13;
      hence thesis by ASYMPT_1:55;
    end;
    thus thesis by ASYMPT_1:56,NEWTON:13;
  end;
