4 4 x + y ---------- 2 / 2 2\ \x + y /
(x^4 + y^4)/(x^2 + y^2)^2
2 2 2*x *y 1 - ----------------- 4 4 2 2 x + y + 2*x *y
1 - 2*x^2*y^2/(x^4 + y^4 + 2*x^2*y^2)