Global and Local Aspects of Exceptional Point
  Exceptional points are singularities of eigenvalues and eigenvectors for complex values of, say, an interaction parameter. They occur universally and are square root branch point singularities of the eigenvalues in the vicinity of level repulsions. The intricate connection between the distribution of exceptional points and particular fluctuation properties of level spacing is discussed. The distribution of the exceptional points of the problem $H_0+\lambda H_1$ is given for the situation of hard chaos. Theoretical predictions of local properties of exceptional points have recently been confirmed experimentally. This relates to the specific topological structure of an exceptional point as well as to the chiral properties of the wave functions associated with exceptional points. 
W. D. Heiss