a(n) = (1 - (-10)^n)/11.

Terms

a(0) =1a(1) =-9a(2) =91a(3) =-909a(4) =9091a(5) =-90909a(6) =909091a(7) =-9090909a(8) =90909091a(9) =-909090909a(10) =9090909091a(11) =-90909090909a(12) =909090909091