a(n) = 2*10^n + 1.

Terms

a(0) =3a(1) =21a(2) =201a(3) =2001a(4) =20001a(5) =200001a(6) =2000001a(7) =20000001a(8) =200000001a(9) =2000000001a(10) =20000000001a(11) =200000000001