I have no idea how to start it, thank you very much!
It should be clear that there is no problem anywhere except possibly at (0,0). What are the partial derivatives? Are they continuous at (0,0)?
To be able to differentiate it must be defined in the point and be continuous, it is defined so we have to watch the limits:
As there is no limit, so the function is no differentiable in (0,0). Right?[/QUOTE]
Yes, excellent point!