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;

theorem
  20 satisfies_Sierpinski_problem_86
  proof
    reconsider a = 3 as Prime by XPRIMES1:3;
    reconsider b = 7 as Prime by XPRIMES1:7;
    reconsider c = 19 as Prime by XPRIMES1:19;
    take a,b,c;
    thus thesis by Th56;
  end;
