Quantal Consequences of Perturbations Which Destroy Structurally   Unstable Orbits in Chaotic Billiards
  Non-generic contributions to the quantal level-density from parallel segments in billiards are investigated. These contributions are due to the existence of marginally stable families of periodic orbits, which are structurally unstable, in the sense that small perturbations, such as a slight tilt of one of the segments, destroy them completely. We investigate the effects of such perturbation on the corresponding quantum spectra, and demonstrate them for the stadium billiard. 
Harel Primack (Department of Physics of Complex Systems, The Weizmann   Institute of Science, 76100 Rehovot, ISRAEL), and Uz...