
theorem
  11 is prime
proof
  now
    let n be Element of NAT;
    11 = 2*5 + 1;
    then
A1: not 2 divides 11 by Th9;
    11 = 3*3 + 2;
    then
A2: not 3 divides 11 by Th9;
    assume 1<n & n*n<=11 & n is prime;
    hence not n divides 11 by A1,A2,Lm3;
  end;
  hence thesis by Th14;
end;
