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

Terms

a(0) =2a(1) =12a(2) =90a(3) =756a(4) =6642a(5) =59292a(6) =532170a(7) =4785156a(8) =43053282a(9) =387440172a(10) =3486843450a(11) =31381236756a(12) =282430067922