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

Terms

a(0) =1a(1) =2a(2) =5a(3) =15a(4) =48a(5) =160a(6) =550a(7) =1937a(8) =6954a(9) =25355a(10) =93633a(11) =349490a(12) =1316397a(13) =4997306a(14) =19100278a(15) =73440718a(16) =283876092a(17) =1102466529a(18) =4299673200a(19) =16832894330a(20) =66127276201a(21) =260595497227