Periodic Orbit Theory of Diffraction
  An extension of the Gutzwiller trace formula is given that includes diffraction effects due to hard wall scatterers or other singularities. The new trace formula involves periodic orbits which have arcs on the surface of singularity and which correspond to creping waves. A new family of resonances in the two disk scattering system can be well described which is completely missing if only the traditional periodic orbits are used. 
G\'abor Vattay (Niels Bohr Institute, Blegdamsvej 17, DK-2100   Copenhagen O, Denmark), Andreas Wirzba (Institut f\"ur Kernp...