a(n) = 4^n - n^12.

Terms

a(0) =1a(1) =3a(2) =-4080a(3) =-531377a(4) =-16776960a(5) =-244139601a(6) =-2176778240a(7) =-13841270817a(8) =-68719411200a(9) =-282429274337a(10) =-999998951424