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

theorem
  21 satisfies_Sierpinski_problem_87a
  proof
    reconsider a = 2 as Prime by XPRIMES1:2;
    reconsider b = 13 as Prime by XPRIMES1:13;
    reconsider c = 17 as Prime by XPRIMES1:17;
    take a,b,c;
    thus thesis by Th50;
  end;
