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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =6a(5) =16a(6) =39a(7) =99a(8) =271a(9) =763a(10) =2146a(11) =6062a(12) =17359a(13) =50337a(14) =147057a(15) =431874a(16) =1275273a(17) =3786649a(18) =11298031a(19) =33846202a(20) =101762937a(21) =306997821a(22) =929038518a(23) =2819426688a(24) =8578433304a(25) =26163061776a(26) =79970186791a(27) =244938841096a(28) =751646959402