a(n) = 7^n - n^6.

Terms

a(0) =1a(1) =6a(2) =-15a(3) =-386a(4) =-1695a(5) =1182a(6) =70993a(7) =705894a(8) =5502657a(9) =39822166a(10) =281475249a(11) =1975555182a(12) =13838301217a(13) =96884183598a(14) =678215543313