a(n) = Sum_{d|n} 7^(d-1).

Terms

a(0) =1a(1) =8a(2) =50a(3) =351a(4) =2402a(5) =16864a(6) =117650a(7) =823894a(8) =5764851a(9) =40356016a(10) =282475250a(11) =1977343950a(12) =13841287202a(13) =96889128064a(14) =678223075300