a(n) = 4*10^n - 1.

Terms

a(0) =3a(1) =39a(2) =399a(3) =3999a(4) =39999a(5) =399999a(6) =3999999a(7) =39999999a(8) =399999999a(9) =3999999999a(10) =39999999999a(11) =399999999999