a(n) = 5^n - n^10.

Terms

a(0) =1a(1) =4a(2) =-999a(3) =-58924a(4) =-1047951a(5) =-9762500a(6) =-60450551a(7) =-282397124a(8) =-1073351199a(9) =-3484831276a(10) =-9990234375a(11) =-25888596476a(12) =-61673223599a(13) =-136637788724a(14) =-283151139351a(15) =-546132812500a(16) =-946923737151a(18) =244230039001