reserve X for set;
reserve a,b,c,k,m,n for Nat;
reserve i,j for Integer;
reserve r,s for Real;
reserve p,p1,p2,p3 for Prime;
reserve z for Complex;

theorem Th19:
  n is a_product_of_two_primes implies n >= 4
  proof
    given p,q being Prime such that
A1: n = p*q;
A2: 4 = 2*2;
    1 < p & 1 < q by INT_2:def 4;
    then 1+1 <= p & 1+1 <= q by NAT_1:13;
    hence thesis by A1,A2,XREAL_1:66;
  end;
