Riordan array (1, 3+3x).

Terms

a(0) =1a(1) =0a(2) =3a(3) =0a(4) =3a(5) =9a(6) =0a(7) =0a(8) =18a(9) =27a(10) =0a(11) =0a(12) =9a(13) =81a(14) =81a(15) =0a(16) =0a(17) =0a(18) =81a(19) =324a(20) =243a(21) =0a(22) =0a(23) =0a(24) =27a(25) =486a(26) =1215a(27) =729a(28) =0a(29) =0