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

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =13a(5) =35a(6) =97a(7) =273a(8) =778a(9) =2240a(10) =6499a(11) =18976a(12) =55703a(13) =164243a(14) =486130a(15) =1443620a(16) =4299365a(17) =12836825a(18) =38413933a(19) =115184282a(20) =346005073a(21) =1041072108a(22) =3137060983a(23) =9465689545a(24) =28596915843a(25) =86492865522a(26) =261876842801a(27) =793661873276