a(n) = 4^n - n^8.

Terms

a(0) =1a(1) =3a(2) =-240a(3) =-6497a(4) =-65280a(5) =-389601a(6) =-1675520a(7) =-5748417a(8) =-16711680a(9) =-42784577a(10) =-98951424a(11) =-210164577a(12) =-413204480a(13) =-748621857a(14) =-1207353600a(15) =-1489148801a(16) =0a(17) =10204111743a(18) =57699516160a(19) =257894343903