Relativistic hodograph equation for a two-dimensional stationary   isentropic hydrodynamical motion
  We derive a relativistic hodograph equation for a two-dimensional stationary isentropic hydrodynamical motion. For the case of stiff matter, when the velocity of sound coincides with the light speed, the singularity in this equation disappears and the solutions become regular in all hodograph plane. 
I. Khalatnikov and A. Kamenshchik