Im searching for books or any other kind of references that cover theorems and techniques to prove if a limit of a multivariable function exists or not.
I know the standard methods to investigate these limits: searching paths that goes to different values, using some variations of L'Hôpital rule, testing Frechet differentiability at the point, basic algebra of continuous functions, etc...
But I dont know any reference in this topic that goes deeper than this. I would like to know something more advanced to the standard techniques and theorems, if there is some material about it.