Viscous Flows and Conditions for Existence of Shocks in Relativistic   Magnetic Hydrodynamics
  We present a criterion for a shock wave existence in relativistic magnetic hydrodynamics with an arbitrary (possibly non-convex) equation of state. The criterion has the form of algebraic inequality that involves equation of state of the fluid; it singles out the physical solutions and it can be easily checked for any discontinuity satisfying concervation laws. The method of proof uses introduction of small viscosity into the coupled set of equations of motion of ideal relativistic fluid with infinite conductivity and Maxwell equations. 
V.I.Zhdanov, P.V. Tytarenko, M.S.Borshch