G.f. A(x) satisfies A(x)^3 = A(x^3) / (1 - 3*x)^3 with A(0)=1.

Terms

a(0) =1a(1) =3a(2) =9a(3) =28a(4) =84a(5) =252a(6) =758a(7) =2274a(8) =6822a(9) =20471a(10) =61413a(11) =184239a(12) =552729a(13) =1658187a(14) =4974561a(15) =14923714a(16) =44771142a(17) =134313426a(18) =402940361a(19) =1208821083a(20) =3626463249a(21) =10879389971a(22) =32638169913a(23) =97914509739a(24) =293743529832a(25) =881230589496