a(n) = 5^n - n^4.

Terms

a(0) =1a(1) =4a(2) =9a(3) =44a(4) =369a(5) =2500a(6) =14329a(7) =75724a(8) =386529a(9) =1946564a(10) =9755625a(11) =48813484a(12) =244119889a(13) =1220674564a(14) =6103477209a(15) =30517527500a(16) =152587825089a(17) =762939369604