a(n) = 8^n + 7.

Terms

a(0) =8a(1) =15a(2) =71a(3) =519a(4) =4103a(5) =32775a(6) =262151a(7) =2097159a(8) =16777223a(9) =134217735a(10) =1073741831a(11) =8589934599a(12) =68719476743a(13) =549755813895