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

Terms

a(0) =1a(1) =0a(2) =0a(3) =1a(4) =1a(5) =1a(6) =5a(7) =9a(8) =13a(9) =39a(10) =87a(11) =157a(12) =389a(13) =923a(14) =1899a(15) =4426a(16) =10582a(17) =23414a(18) =54022a(19) =128643a(20) =295735a(21) =686881a(22) =1631513a(23) =3825456a(24) =8974024a(25) =21330400a(26) =50550032a(27) =119644037a(28) =285176865a(29) =680215735