a(n) = (20^n - 1)/19.

Terms

a(0) =0a(1) =1a(2) =21a(3) =421a(4) =8421a(5) =168421a(6) =3368421a(7) =67368421a(8) =1347368421a(9) =26947368421a(10) =538947368421