
theorem Th28:
  13 is prime
proof
  now
    let n be Element of NAT;
    13 = 2*6 + 1;
    then
A1: not 2 divides 13 by Th9;
    13 = 3*4 + 1;
    then
A2: not 3 divides 13 by Th9;
    assume 1<n & n*n<=13 & n is prime;
    hence not n divides 13 by A1,A2,Lm3;
  end;
  hence thesis by Th14;
end;
