On a generalization of l'Hopital's rule for multivariable functions
  For students and their lecturers and instructors interested in the natural problem of a possible generalization of l'Hopital's rule for functions depending on two or more variables, we offer our approach. For instructors, we discuss the technique of constructing indeterminate forms at a given point and having a given double limit. 
V. V. Ivlev, I. A. Shilin
math.HO