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

Terms

a(0) =1a(1) =7a(2) =60a(3) =503a(4) =4080a(5) =32743a(6) =262108a(7) =2097103a(8) =16777152a(9) =134217647a(10) =1073741724a(11) =8589934471a(12) =68719476592a(13) =549755813719