a(n) = 5^n - n^12.

Terms

a(0) =1a(1) =4a(2) =-4071a(3) =-531316a(4) =-16776591a(5) =-244137500a(6) =-2176766711a(7) =-13841209076a(8) =-68719086111a(9) =-282427583356a(10) =-999990234375