“…Yet the well-posedness theory of multi-dimensional problems becomes much more complicated and challenging in the presence of vacuum (that is, the flow density is zero) despite its fundamental importance both physically and theoretically in understanding the behaviors of the solutions of compressible viscous flows. Substantial difficulties arise due to the strong degeneracy of the hyperbolic-parabolic system in the case where a vacuum state appears (see [3,16,22,25,29,30,38,40]).…”