a(n) = 2*(a(n-1) + a(n-2) + a(n-3)) - a(n-4) for n >= 4, with initial terms 0,1,0,1.

Terms

a(0) =0a(1) =1a(2) =0a(3) =1a(4) =4a(5) =9a(6) =28a(7) =81a(8) =232a(9) =673a(10) =1944a(11) =5617a(12) =16236a(13) =46921a(14) =135604a(15) =391905a(16) =1132624a(17) =3273345a(18) =9460144a(19) =27340321a(20) =79014996a(21) =228357577a(22) =659965644a(23) =1907336113a(24) =5512303672a(25) =15930853281a(26) =46041020488a(27) =133061018769a(28) =384553481404