a(n) = 5*(10^n - 1)/9.

Terms

a(0) =0a(1) =5a(2) =55a(3) =555a(4) =5555a(5) =55555a(6) =555555a(7) =5555555a(8) =55555555a(9) =555555555a(10) =5555555555a(11) =55555555555a(12) =555555555555