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

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =19a(5) =51a(6) =114a(7) =312a(8) =981a(9) =2616a(10) =6564a(11) =19647a(12) =59922a(13) =159056a(14) =430302a(15) =1329996a(16) =3926217a(17) =10498968a(18) =30052851a(19) =93244764a(20) =267690168a(21) =729649143a(22) =2173840338a(23) =6663260223a(24) =18768583674a(25) =52570016676a(26) =160362713250a(27) =481809941520