a(n) = 9^n - n^4.

Terms

a(0) =1a(1) =8a(2) =65a(3) =648a(4) =6305a(5) =58424a(6) =530145a(7) =4780568a(8) =43042625a(9) =387413928a(10) =3486774401a(11) =31381044968a(12) =282429515745