a(n) = 7^n - n.

Terms

a(0) =1a(1) =6a(2) =47a(3) =340a(4) =2397a(5) =16802a(6) =117643a(7) =823536a(8) =5764793a(9) =40353598a(10) =282475239a(11) =1977326732a(12) =13841287189a(13) =96889010394a(14) =678223072835