a(n) = 9*(10^n - 1).

Terms

a(0) =0a(1) =81a(2) =891a(3) =8991a(4) =89991a(5) =899991a(6) =8999991a(7) =89999991a(8) =899999991a(9) =8999999991a(10) =89999999991a(11) =899999999991