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

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =8a(5) =25a(6) =80a(7) =267a(8) =911a(9) =3170a(10) =11192a(11) =39993a(12) =144320a(13) =525124a(14) =1924196a(15) =7093603a(16) =26288928a(17) =97878831a(18) =365918064a(19) =1372982706a(20) =5168555770a(21) =19514482964a(22) =73876936272a(23) =280363191353