a(n) = 8*7^n+1.

Terms

a(0) =9a(1) =57a(2) =393a(3) =2745a(4) =19209a(5) =134457a(6) =941193a(7) =6588345a(8) =46118409a(9) =322828857a(10) =2259801993a(11) =15818613945a(12) =110730297609a(13) =775112083257