Consider the second order Partial Differential Equation (PDE)
\[
4x^2 \frac{\partial^2 u}{\partial x^2} + 4(x + y) \frac{\partial^2 u}{\partial x \partial y} + (x^2 + y^2) \frac{\partial^2 u}{\partial y^2} - u = 0.
\]
Then which one of the following statements is correct?