
theorem
  53 is prime
proof
  now
    53 = 2*26 + 1; hence not 2 divides 53 by NAT_4:9;
    53 = 3*17 + 2; hence not 3 divides 53 by NAT_4:9;
    53 = 5*10 + 3; hence not 5 divides 53 by NAT_4:9;
    53 = 7*7 + 4; hence not 7 divides 53 by NAT_4:9;
  end;
  then for n being Element of NAT st 1 < n & n*n <= 53 & n is prime
  holds not n divides 53 by XPRIMET1:8;
  hence thesis by NAT_4:14;
end;
