a(n) = 4^n - n^11.

Terms

a(0) =1a(1) =3a(2) =-2032a(3) =-177083a(4) =-4194048a(5) =-48827101a(6) =-362792960a(7) =-1977310359a(8) =-8589869056a(9) =-31380797465a(10) =-99998951424a(11) =-285307476307a(12) =-742991593472