a(n) = 2*11^n-1.

Terms

a(0) =1a(1) =21a(2) =241a(3) =2661a(4) =29281a(5) =322101a(6) =3543121a(7) =38974341a(8) =428717761a(9) =4715895381a(10) =51874849201a(11) =570623341221