a(n) = 8^n - 2^n + 1^n.

Terms

a(0) =1a(1) =7a(2) =61a(3) =505a(4) =4081a(5) =32737a(6) =262081a(7) =2097025a(8) =16776961a(9) =134217217a(10) =1073740801a(11) =8589932545a(12) =68719472641a(13) =549755805697