a(n) = 2^(n mod 2)*5*10^floor(n/2) - 1.

Terms

a(0) =4a(1) =9a(2) =49a(3) =99a(4) =499a(5) =999a(6) =4999a(7) =9999a(8) =49999a(9) =99999a(10) =499999a(11) =999999a(12) =4999999a(13) =9999999a(14) =49999999a(15) =99999999a(16) =499999999a(17) =999999999a(18) =4999999999a(19) =9999999999a(20) =49999999999a(21) =99999999999a(22) =499999999999a(23) =999999999999