a(n) = 9^n - n^3.

Terms

a(0) =1a(1) =8a(2) =73a(3) =702a(4) =6497a(5) =58924a(6) =531225a(7) =4782626a(8) =43046209a(9) =387419760a(10) =3486783401a(11) =31381058278a(12) =282429534753