a(n) = 10^n - n^4.

Terms

a(0) =1a(1) =9a(2) =84a(3) =919a(4) =9744a(5) =99375a(6) =998704a(7) =9997599a(8) =99995904a(9) =999993439a(10) =9999990000a(11) =99999985359a(12) =999999979264