Trace formulas and spectral statistics of diffractive systems
  Diffractive systems are quantum-mechanical models with point-like singularities where usual semiclassical approximation breaks down. An overview of recent investigations of such systems is presented. The following examples are considered in details: (i) billiards (both integrable and chaotic) with small-size scatterers, (ii) pseudo-integrable polygonal plane billiards, and (iii) billiards with the Bohr-Aharonov flux lines. First, the diffractive trace formulas are discussed with particular emphasis on models where the diffractive coefficient diverges in certain directions. Second, it is demonstrated that the spectral statistics of diffractive models are different from the statistics of both integrable and chaotic systems. The main part of the paper is devoted to analytical calculations of spectral statistics for certain diffractive models. 
E.Bogomolny