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

Terms

a(0) =1a(1) =4a(2) =-7a(3) =-118a(4) =-399a(5) =0a(6) =7849a(7) =61318a(8) =357857a(9) =1894076a(10) =9665625a(11) =48667074a(12) =243891793a(13) =1220331832a(14) =6102977801a(15) =30516818750a(16) =152586842049a(17) =762938033268