(* Factors of x^20 - 1 *)
  (-1 + x)*(1 + x)*(1 + x^2)*(1 - x + x^2 - x^3 + x^4)*
   (1 + x + x^2 + x^3 + x^4)*(1 - x^2 + x^4 - x^6 + x^8)
