
theorem Th26:
  7 is prime
proof
  now
    let n be Element of NAT;
    7 = 2*3 + 1;
    then
A1: not 2 divides 7 by Th9;
    7 = 3*2 + 1;
    then
A2: not 3 divides 7 by Th9;
    assume 1<n & n*n<=7 & n is prime;
    hence not n divides 7 by A1,A2,Lm3;
  end;
  hence thesis by Th14;
end;
