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

Terms

a(0) =0a(1) =11a(2) =99a(3) =1001a(4) =9999a(5) =100001a(6) =999999a(7) =10000001a(8) =99999999a(9) =1000000001a(10) =9999999999a(11) =100000000001a(12) =999999999999