While I was doing exercises about this, I noticed that, for example f(x,y)=e^{x^2+y^2} was very similar to f(x,y)=x^2+y^2
And something similar with:
f(x,y)=e^{(1-x^2-y^2)^{(1/2)}} and f(x,y)=(1-x^2-y^2)^{(1/2)}
So I supposed that some transformations that preserves some properties kinda...