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

Terms

a(0) =1a(1) =1a(2) =4a(3) =16a(4) =71a(5) =336a(6) =1660a(7) =8464a(8) =44207a(9) =235306a(10) =1271807a(11) =6961307a(12) =38508659a(13) =214950425a(14) =1209170536a(15) =6848080767a(16) =39014400171a(17) =223439516338