  The question of the shape of the density PDF for supersonic turbulence is
addressed, using both analytical and numerical methods. For isothermal
supersonic turbulence, the PDF is Log-Normal, with a width that scales
approximately linearly with the Mach number. For a polytropic equation of
state, with an effective gamma smaller than one, the PDF becomes skewed and
reminiscent of (but not equal to) a power law on the high density side.
