a(n) = (8*9^n + (-9)^n)/9.

Terms

a(0) =1a(1) =7a(2) =81a(3) =567a(4) =6561a(5) =45927a(6) =531441a(7) =3720087a(8) =43046721a(9) =301327047a(10) =3486784401a(11) =24407490807a(12) =282429536481