a(n) = (2+(-5)^n)/3.

Terms

a(0) =1a(1) =-1a(2) =9a(3) =-41a(4) =209a(5) =-1041a(6) =5209a(7) =-26041a(8) =130209a(9) =-651041a(10) =3255209a(11) =-16276041a(12) =81380209a(13) =-406901041a(14) =2034505209a(15) =-10172526041a(16) =50862630209a(17) =-254313151041