a(n) = 5^n - n^9.

Terms

a(0) =1a(1) =4a(2) =-487a(3) =-19558a(4) =-261519a(5) =-1950000a(6) =-10062071a(7) =-40275482a(8) =-133827103a(9) =-385467364a(10) =-990234375a(11) =-2309119566a(12) =-4915639727a(13) =-9383796248a(14) =-14557531159a(15) =-7925781250a(16) =83868413889a(17) =644351576628