a(n) = 7^n - n^3.

Terms

a(0) =1a(1) =6a(2) =41a(3) =316a(4) =2337a(5) =16682a(6) =117433a(7) =823200a(8) =5764289a(9) =40352878a(10) =282474249a(11) =1977325412a(12) =13841285473a(13) =96889008210a(14) =678223070105