a(n) = 8^n - n^4.

Terms

a(0) =1a(1) =7a(2) =48a(3) =431a(4) =3840a(5) =32143a(6) =260848a(7) =2094751a(8) =16773120a(9) =134211167a(10) =1073731824a(11) =8589919951a(12) =68719456000a(13) =549755785327