7^n - 3^n + 1.

Terms

a(0) =1a(1) =5a(2) =41a(3) =317a(4) =2321a(5) =16565a(6) =116921a(7) =821357a(8) =5758241a(9) =40333925a(10) =282416201a(11) =1977149597a(12) =13840755761a(13) =96887416085a(14) =678218289881