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

Terms

a(0) =8a(1) =71a(2) =575a(3) =4607a(4) =36863a(5) =294911a(6) =2359295a(7) =18874367a(8) =150994943a(9) =1207959551a(10) =9663676415a(11) =77309411327a(12) =618475290623