a(n) = 7^n - n^4.

Terms

a(0) =1a(1) =6a(2) =33a(3) =262a(4) =2145a(5) =16182a(6) =116353a(7) =821142a(8) =5760705a(9) =40347046a(10) =282465249a(11) =1977312102a(12) =13841266465a(13) =96888981846a(14) =678223034433