A characterization of Herglotz-Nevanlinna functions in two variables via   integral representations
  We derive an integral representation for Herglotz-Nevanlinna functions in two variables which provides a complete characterization of this class in terms of a real number, two non-negative numbers and a positive measure satisfying certain conditions. Further properties of the representing measures are discussed. 
Annemarie Luger, Mitja Nedic
math.CV