Find $\left(\frac{\partial^{2} f}{\partial x^{2}}\right)\left(\frac{\partial^{2} f}{\partial y^{2}}\right)-\left(\frac{\partial^{2} f}{\partial x \partial y}\right)^{2}$ for the following functions:
a $f(x, y)=x^{3}+y^{3}-3 x y$
b $f(x, y)=2 x y+\frac{2000}{x}+\frac{2000}{y}$