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

Terms

a(0) =1a(1) =2a(2) =5a(3) =18a(4) =70a(5) =294a(6) =1291a(7) =5864a(8) =27314a(9) =129766a(10) =626367a(11) =3063096a(12) =15143562a(13) =75563924a(14) =380062186a(15) =1924840480a(16) =9807649900a(17) =50241194250a(18) =258597717591