  We consider the Riemann problem for relativistic flows of polytropic fluids
and find relations for the flow characteristics. Evolution of physical
quantities take especially simple form for the case of cold magnetized plasmas.
We find exact, explicit analytical solutions for one dimensional expansion of
magnetized plasma into vacuum, valid for arbitrary magnetization. We also
consider expansion into cold unmagnetized external medium both for stationary
initial conditions and for initially moving plasma, as well as reflection of
rarefaction wave from a wall. We also find self-similar structure of
three-dimensional magnetized outflows into vacuum, valid close to the
plasma-vacuum interface.
  The key results of this work, the self-similar solutions, were incorporated
post-initial submission into appendices of the published version of Granot et
al. (2010).
