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

Terms

a(0) =1a(1) =3a(2) =0a(3) =-17a(4) =0a(5) =399a(6) =2800a(7) =13983a(8) =61440a(9) =255583a(10) =1038576a(11) =4179663a(12) =16756480a(13) =67080303a(14) =268397040a(15) =1073691199a(16) =4294901760a(17) =17179785663a(18) =68719371760a(19) =274877776623