a(n) = 4^n - n^5.

Terms

a(0) =1a(1) =3a(2) =-16a(3) =-179a(4) =-768a(5) =-2101a(6) =-3680a(7) =-423a(8) =32768a(9) =203095a(10) =948576a(11) =4033253a(12) =16528384a(13) =66737571a(14) =267897632a(15) =1072982449a(16) =4293918720a(17) =17178449327a(18) =68717587168a(19) =274875430845