$\begin{array}{1 1} (x^4+y^4)(x^2+y^2)(x^2-y^2) \\ 2a(1-4a^2)(1-2a)(1+2a) \\ (x^4+y^4)(x^2-y^2) \\ (x^2+y^2)(x^2-y^2) \end{array} $
