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

Terms

a(0) =1a(1) =1a(2) =3a(3) =9a(4) =31a(5) =114a(6) =438a(7) =1739a(8) =7077a(9) =29364a(10) =123756a(11) =528324a(12) =2279868a(13) =9928679a(14) =43580301a(15) =192601419a(16) =856317717a(17) =3827501985a(18) =17188943523a(19) =77521747638a(20) =350959738842