a(n) = 2*a(n-1) + 2*a(n-2) + a(n-3). a(0) = -1, a(1) = 1, a(2) = 1.

Terms

a(0) =-1a(1) =1a(2) =1a(3) =3a(4) =9a(5) =25a(6) =71a(7) =201a(8) =569a(9) =1611a(10) =4561a(11) =12913a(12) =36559a(13) =103505a(14) =293041a(15) =829651a(16) =2348889a(17) =6650121a(18) =18827671a(19) =53304473a(20) =150914409a(21) =427265435a(22) =1209664161a(23) =3424773601a(24) =9696140959a(25) =27451493281a(26) =77720042081a(27) =220039211683a(28) =622970000809