a(n) = 3*a(n-1) - 2*a(n-2), with a(1) = 9, a(2) = 12.

Terms

a(0) =9a(1) =12a(2) =18a(3) =30a(4) =54a(5) =102a(6) =198a(7) =390a(8) =774a(9) =1542a(10) =3078a(11) =6150a(12) =12294a(13) =24582a(14) =49158a(15) =98310a(16) =196614a(17) =393222a(18) =786438a(19) =1572870a(20) =3145734a(21) =6291462a(22) =12582918a(23) =25165830a(24) =50331654a(25) =100663302a(26) =201326598a(27) =402653190a(28) =805306374a(29) =1610612742