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

Terms

a(0) =1a(1) =4a(2) =15a(3) =94a(4) =706a(5) =5769a(6) =49923a(7) =449376a(8) =4164228a(9) =39459852a(10) =380594767a(11) =3724049805a(12) =36876008673a(13) =368835076813