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

Terms

a(0) =1a(1) =3a(2) =15a(3) =82a(4) =477a(5) =2901a(6) =18235a(7) =117555a(8) =773085a(9) =5166478a(10) =34987170a(11) =239570655a(12) =1655933060a(13) =11538839130a(14) =80971109712a(15) =571702698185