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

Terms

a(0) =1a(1) =1a(2) =4a(3) =13a(4) =50a(5) =201a(6) =841a(7) =3627a(8) =15993a(9) =71803a(10) =327082a(11) =1508002a(12) =7023446a(13) =32995626a(14) =156173668a(15) =744029238a(16) =3565030063a(17) =17169013899a(18) =83061503584a(19) =403483653745