  The relativistic hydrodynamic system of equations for a perfect fluid obeying
a causal equation of state is hyperbolic (Anile 1989 {\it Relativistic Fluids
and Magneto-Fluids} (Cambridge: Cambridge University Press)). In this report,
we derive the conditions for this system to be convex in terms of the
fundamental derivative of the equation of state (Menikoff and Plohr 1989 {\it
Rev. Mod. Phys.} {\bf 61} 75). The classical limit is recovered.
