X^4 - x^3 - x + 1 = x^3 (x - 1) - 1(x - 1) = (x^3 - 1)(x - 1) = (x - 1)(x^2 + x + 1)(x - 1) = (x - 1)^2(x^2 + x + 1)
X^4 - x^3 - x + 1 = x^3 (x - 1) - 1(x - 1) = (x^3 - 1)(x - 1) = (x - 1)(x^2 + x + 1)(x - 1) = (x - 1)^2(x^2 + x + 1)