Perhaps your teacher was referring to this (or some other) demonstration:
http://demonstrations.wolfram.com/MorseSmaleFlowsOnATiltedTorus/
Thank you, I saw it, but actually, it's not the thing, I'm asking about... I need the way to find the change of variables, to see this change and write it like an explicit function.
Please consider to deliver a slight bit more information to the community forum than the teacher did deliver to you.
x y = -1
f[x_,y_] := 1/(1 + x y)
{x,y}={0,0}
then there are entries like Hessian which could be used once the task at hand is clarified.
