a(n) = 6^n - n^12.

Terms

a(0) =1a(1) =5a(2) =-4060a(3) =-531225a(4) =-16775920a(5) =-244132849a(6) =-2176735680a(7) =-13841007265a(8) =-68717797120a(9) =-282419458785a(10) =-999939533824