a(n) = 4^n - n^7.

Terms

a(0) =1a(1) =3a(2) =-112a(3) =-2123a(4) =-16128a(5) =-77101a(6) =-275840a(7) =-807159a(8) =-2031616a(9) =-4520825a(10) =-8951424a(11) =-15292867a(12) =-19054592a(13) =4360347a(14) =163021952a(15) =902882449a(16) =4026531840a(17) =16769530511a(18) =68107256704a(19) =273984035205