G.f. satisfies A(x) = 1 + 2*x + x^3*A(x)^3.

Terms

a(0) =1a(1) =2a(2) =0a(3) =1a(4) =6a(5) =12a(6) =11a(7) =30a(8) =120a(9) =252a(10) =408a(11) =1104a(12) =3415a(13) =7710a(14) =15984a(15) =42609a(16) =118422a(17) =281820a(18) =657468a(19) =1732008a(20) =4608288a(21) =11375240a(22) =28100400a(23) =73598112a(24) =192776895a(25) =487507038a(26) =1239516384a(27) =3241480275a(28) =8460205842a(29) =21733941060