a(n) = 4^n-n^9.

Terms

a(0) =1a(1) =3a(2) =-496a(3) =-19619a(4) =-261888a(5) =-1952101a(6) =-10073600a(7) =-40337223a(8) =-134152192a(9) =-387158345a(10) =-998951424a(11) =-2353753387a(12) =-5143003136a(13) =-10537390509a(14) =-20392611328a(15) =-37369617551a(16) =-64424509440a(17) =-101408007313a(18) =-129639813632