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

Terms

a(0) =1a(1) =3a(2) =9a(3) =34a(4) =141a(5) =621a(6) =2849a(7) =13467a(8) =65127a(9) =320686a(10) =1602294a(11) =8103087a(12) =41397186a(13) =213331026a(14) =1107604764a(15) =5788249329a(16) =30422897664a(17) =160717169622a(18) =852894534042