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

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =24a(5) =80a(6) =278a(7) =997a(8) =3670a(9) =13782a(10) =52588a(11) =203314a(12) =794726a(13) =3135540a(14) =12470444a(15) =49942305a(16) =201233170a(17) =815205699a(18) =3318291966a(19) =13565162636a(20) =55669063762a(21) =229257178198a(22) =947142023262