a(n) = 5^n - n^7.

Terms

a(0) =1a(1) =4a(2) =-103a(3) =-2062a(4) =-15759a(5) =-75000a(6) =-264311a(7) =-745418a(8) =-1706527a(9) =-2829844a(10) =-234375a(11) =29340954a(12) =208308817a(13) =1157954608a(14) =5998102121a(15) =30346718750a(16) =152319455169a(17) =762529114452