x^4-4x^3+4x^2-1
=x^2(x^2-4x+4)-1
=x^2(x-2)^2-1
=[x(x-2)]^2-1^2
=[x(x-2)+1][x(x-2)-1]
=(x^2-2x+1)(x^2-2x-1)
=(x-1)^2(x-1-√2)(x-1+√2)