G.f.: 1/(1-(2*x^2+x^3)*(1-sqrt(1-4*x))).

Terms

a(0) =1a(1) =0a(2) =0a(3) =4a(4) =6a(5) =10a(6) =40a(7) =114a(8) =312a(9) =988a(10) =3184a(11) =10330a(12) =34516a(13) =117620a(14) =406204a(15) =1421132a(16) =5027504a(17) =17950224a(18) =64605044a(19) =234157026a(20) =853907268a(21) =3130879580a(22) =11534945108a(23) =42681346612a(24) =158542854496a(25) =590990616384