a(n) = 10*8^n - 1.

Terms

a(0) =9a(1) =79a(2) =639a(3) =5119a(4) =40959a(5) =327679a(6) =2621439a(7) =20971519a(8) =167772159a(9) =1342177279a(10) =10737418239a(11) =85899345919a(12) =687194767359