a(n) = 9^n+3^n-1.

Terms

a(0) =1a(1) =11a(2) =89a(3) =755a(4) =6641a(5) =59291a(6) =532169a(7) =4785155a(8) =43053281a(9) =387440171a(10) =3486843449a(11) =31381236755a(12) =282430067921