Statistical properties of periodic orbits in 4-disk billiard system:   pruning-proof property
  Periodic orbit theory for classical hyperbolic system is very significant matter of how we can interpret spectral statistics in terms of semiclassical theory. Although pruning is significant and generic property for almost all hyperbolic systems, pruning-proof property for the correlation among the periodic orbits which gains a resurgence of second term of the random matrix form factor remains open problem. In the light of the semiclassical form factor, our attention is paid to statistics for the pairs of periodic orbits. Also in the context of pruning, we investigated statistical properties of the "actual" periodic orbits in 4-disk billiard system. This analysis presents some universality for pair-orbits' statistics. That is, even if the pruning progresses, there remains the periodic peak structure in the statistics for periodic orbit pairs. From that property, we claim that if the periodic peak structure contributes to the correlation, namely the off-diagonal part of the semiclassical form factor, then the correlation must remain while pruning progresse. 
Takeshi Asamizuya