a(n) = 3*10^n + 1.

Terms

a(0) =4a(1) =31a(2) =301a(3) =3001a(4) =30001a(5) =300001a(6) =3000001a(7) =30000001a(8) =300000001a(9) =3000000001a(10) =30000000001a(11) =300000000001