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

Terms

a(0) =1a(1) =1a(2) =3a(3) =15a(4) =82a(5) =495a(6) =3147a(7) =20812a(8) =141621a(9) =985287a(10) =6976369a(11) =50108232a(12) =364202415a(13) =2673756449a(14) =19797659586a(15) =147677816532