a(n) = 2^n + 7^n.

Terms

a(0) =2a(1) =9a(2) =53a(3) =351a(4) =2417a(5) =16839a(6) =117713a(7) =823671a(8) =5765057a(9) =40354119a(10) =282476273a(11) =1977328791a(12) =13841291297a(13) =96889018599a(14) =678223089233