a(n) = 3^(n-1) - 2^n.

Terms

a(0) =-1a(1) =-1a(2) =1a(3) =11a(4) =49a(5) =179a(6) =601a(7) =1931a(8) =6049a(9) =18659a(10) =57001a(11) =173051a(12) =523249a(13) =1577939a(14) =4750201a(15) =14283371a(16) =42915649a(17) =128878019a(18) =386896201a(19) =1161212891a(20) =3484687249a(21) =10456158899a(22) =31372671001a(23) =94126401611a(24) =282395982049a(25) =847221500579