a(n) = 8*10^n - 1.

Terms

a(0) =7a(1) =79a(2) =799a(3) =7999a(4) =79999a(5) =799999a(6) =7999999a(7) =79999999a(8) =799999999a(9) =7999999999a(10) =79999999999a(11) =799999999999