a(n) = 7*10^n - 1.

Terms

a(0) =6a(1) =69a(2) =699a(3) =6999a(4) =69999a(5) =699999a(6) =6999999a(7) =69999999a(8) =699999999a(9) =6999999999a(10) =69999999999a(11) =699999999999