a(n) = 7^n - 7*n.

Terms

a(0) =1a(1) =0a(2) =35a(3) =322a(4) =2373a(5) =16772a(6) =117607a(7) =823494a(8) =5764745a(9) =40353544a(10) =282475179a(11) =1977326666a(12) =13841287117a(13) =96889010316a(14) =678223072751