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

Terms

a(0) =-1a(1) =4a(2) =-13a(3) =49a(4) =-181a(5) =676a(6) =-2521a(7) =9409a(8) =-35113a(9) =131044a(10) =-489061a(11) =1825201a(12) =-6811741a(13) =25421764a(14) =-94875313a(15) =354079489a(16) =-1321442641a(17) =4931691076a(18) =-18405321661a(19) =68689595569a(20) =-256353060613a(21) =956722646884