a(n) = 7^n + 2^n - 1.

Terms

a(0) =1a(1) =8a(2) =52a(3) =350a(4) =2416a(5) =16838a(6) =117712a(7) =823670a(8) =5765056a(9) =40354118a(10) =282476272a(11) =1977328790a(12) =13841291296a(13) =96889018598a(14) =678223089232