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) =56a(5) =279a(6) =1464a(7) =7973a(8) =44637a(9) =255315a(10) =1485530a(11) =8764851a(12) =52317291a(13) =315355879a(14) =1916894598a(15) =11736741483a(16) =72318894814a(17) =448109853543