Ultra-Relativistic Expansion of Ideal Fluid with Linear Equation of   State
  We study solutions of the relativistic hydrodynamical equations, which describe spherical or cylindrical expansion of ideal fluid. We derived approximate solutions involving two arbitrary functions, which describe asymptotic behavior of expanding fireballs in ultra-relativistic limit. In case of a sufficiently stiff linear equation of state the solution may be represented in form of an asymptotic series. 
V.I.Zhdanov, M.S.Borshch