a(n) = 9^n + 5^n - 1.

Terms

a(0) =1a(1) =13a(2) =105a(3) =853a(4) =7185a(5) =62173a(6) =547065a(7) =4861093a(8) =43437345a(9) =389373613a(10) =3496550025a(11) =31429887733a(12) =282673677105