G.f.: (1 + 7*x) / (1 - 2*x - 9*x^2).

Terms

a(0) =1a(1) =9a(2) =27a(3) =135a(4) =513a(5) =2241a(6) =9099a(7) =38367a(8) =158625a(9) =662553a(10) =2752731a(11) =11468439a(12) =47711457a(13) =198638865a(14) =826680843a(15) =3441111471a(16) =14322350529a(17) =59614704297a(18) =248130563355