G.f. satisfies A(x) = 1 + x*(1 + x*A(x)^4)^2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =9a(4) =36a(5) =172a(6) =842a(7) =4310a(8) =22676a(9) =121896a(10) =666884a(11) =3699973a(12) =20771096a(13) =117765084a(14) =673367034a(15) =3878538930a(16) =22483446152a(17) =131070712924a(18) =767929882240