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

Terms

a(0) =1a(1) =8a(2) =32a(3) =120a(4) =448a(5) =1672a(6) =6240a(7) =23288a(8) =86912a(9) =324360a(10) =1210528a(11) =4517752a(12) =16860480a(13) =62924168a(14) =234836192a(15) =876420600a(16) =3270846208a(17) =12206964232a(18) =45557010720a(19) =170021078648a(20) =634527303872