a(n) = 7^n - n^5.

Terms

a(0) =1a(1) =6a(2) =17a(3) =100a(4) =1377a(5) =13682a(6) =109873a(7) =806736a(8) =5732033a(9) =40294558a(10) =282375249a(11) =1977165692a(12) =13841038369a(13) =96888639114a(14) =678222535025