a(n) = 3^n - n^12.

Terms

a(0) =1a(1) =2a(2) =-4087a(3) =-531414a(4) =-16777135a(5) =-244140382a(6) =-2176781607a(7) =-13841285014a(8) =-68719470175a(9) =-282429516798a(10) =-999999940951