a(n) = 8^n - n.

Terms

a(0) =1a(1) =7a(2) =62a(3) =509a(4) =4092a(5) =32763a(6) =262138a(7) =2097145a(8) =16777208a(9) =134217719a(10) =1073741814a(11) =8589934581a(12) =68719476724a(13) =549755813875