a(n) = Sum_{d|n} phi(d)*7^(n/d).

Terms

a(0) =0a(1) =7a(2) =56a(3) =357a(4) =2464a(5) =16835a(6) =118104a(7) =823585a(8) =5767328a(9) =40354335a(10) =282492280a(11) =1977326813a(12) =13841410464a(13) =96889010491a(14) =678223896728