a(n) = 8^n - 1.

Terms

a(0) =0a(1) =7a(2) =63a(3) =511a(4) =4095a(5) =32767a(6) =262143a(7) =2097151a(8) =16777215a(9) =134217727a(10) =1073741823a(11) =8589934591a(12) =68719476735a(13) =549755813887