a(n) = Sum_{k=2..n} 9^k.

Terms

a(0) =0a(1) =81a(2) =810a(3) =7371a(4) =66420a(5) =597861a(6) =5380830a(7) =48427551a(8) =435848040a(9) =3922632441a(10) =35303692050a(11) =317733228531