a(n) = 8^n-7.

Terms

a(0) =1a(1) =57a(2) =505a(3) =4089a(4) =32761a(5) =262137a(6) =2097145a(7) =16777209a(8) =134217721a(9) =1073741817a(10) =8589934585a(11) =68719476729a(12) =549755813881