a(n) = 10^n - (5^(2^n) mod 10^n).

Terms

a(0) =5a(1) =75a(2) =375a(3) =9375a(4) =9375a(5) =109375a(6) =7109375a(7) =87109375a(8) =787109375a(9) =1787109375a(10) =81787109375a(11) =81787109375a(12) =81787109375