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

Terms

a(0) =1a(1) =2a(2) =3a(3) =8a(4) =23a(5) =72a(6) =237a(7) =808a(8) =2830a(9) =10118a(10) =36779a(11) =135510a(12) =504935a(13) =1899494a(14) =7204238a(15) =27517766a(16) =105761937a(17) =408715018a(18) =1587169591a(19) =6190357852a(20) =24238696551a(21) =95244997612a(22) =375469654543