a(n) = (9^n + 5^n)/2.

Terms

a(0) =1a(1) =7a(2) =53a(3) =427a(4) =3593a(5) =31087a(6) =273533a(7) =2430547a(8) =21718673a(9) =194686807a(10) =1748275013a(11) =15714943867a(12) =141336838553