  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.

Chaotic Dynamics;Condensed Matter;Chaotic Dynamics