a(n) = 5^n + 7^n.

Terms

a(0) =2a(1) =12a(2) =74a(3) =468a(4) =3026a(5) =19932a(6) =133274a(7) =901668a(8) =6155426a(9) =42306732a(10) =292240874a(11) =2026154868a(12) =14085427826a(13) =98109713532a(14) =684326588474