The associated factors of this poly are (x-1), (x+1), (x - i) and (x+i).
Multiply these out. Note that (x-1)(x+1) = x^2 - 1, and that
(x-i)(x+i) = x^2 + 1. So, mult. (x^2 -1) and (x^2 + 1) together. Arrange the resulting poly so that powers of x are in descending order, beginning with x^4.
