Discontinuities in Relativistic Hydrodynamics Across Space-like and   Time-like Hypersurfaces
  Discontinuities in relativistic hydrodynamics - shock waves and freeze-out shocks - are considered across both space-like and time-like hypersurfaces. We analyze the peculiar features of the freeze-out discontinuities and their connection to the shock-wave phenomena. 
K. A. Bugaev, M. I. Gorenstein and W. Greiner