G.f. A(x) satisfies: A(x - 3*x^3) = 1/(1 - 3*x).

Terms

a(0) =1a(1) =3a(2) =9a(3) =36a(4) =135a(5) =567a(6) =2268a(7) =9720a(8) =40095a(9) =173745a(10) =729729a(11) =3184272a(12) =13533156a(13) =59337684a(14) =254304360a(15) =1118939184a(16) =4825425231a(17) =21288640725a(18) =92250776475a(19) =407845538100