Question

How to prove a multivariable function is not differentiable?

Answer 1

The function f is differentiable on a region R if it is differentiable at each point of R. Informally, f (x,y) is differentiability at (a,b) if it is “well approximated” by a linear function L(x,y) near (a,b) If f (x,y) is differentiable, then partial derivatives must exist