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

Terms

a(0) =1a(1) =1a(2) =3a(3) =12a(4) =54a(5) =267a(6) =1392a(7) =7533a(8) =41907a(9) =238179a(10) =1377000a(11) =8072619a(12) =47876901a(13) =286739379a(14) =1731753408a(15) =10534970955a(16) =64495997604a(17) =397062007827