a(n) = 8*9^n-1.

Terms

a(0) =7a(1) =71a(2) =647a(3) =5831a(4) =52487a(5) =472391a(6) =4251527a(7) =38263751a(8) =344373767a(9) =3099363911a(10) =27894275207a(11) =251048476871