a(n) = 7^n-1.

Terms

a(0) =0a(1) =6a(2) =48a(3) =342a(4) =2400a(5) =16806a(6) =117648a(7) =823542a(8) =5764800a(9) =40353606a(10) =282475248a(11) =1977326742a(12) =13841287200a(13) =96889010406a(14) =678223072848