a(n) = 5^n - n^6.

Terms

a(0) =1a(1) =4a(2) =-39a(3) =-604a(4) =-3471a(5) =-12500a(6) =-31031a(7) =-39524a(8) =128481a(9) =1421684a(10) =8765625a(11) =47056564a(12) =241154641a(13) =1215876316a(14) =6095986089a(15) =30506187500a(16) =152571113409a(17) =762915315556