Singularity formation for compressible Euler equations
  It is well-known that shock will form in finite time for hyperbolic conservation laws from initial nonlinear compression no matter how small and smooth the data are. Classical results, including Lax [14], Liu [22], Li-Zhou-Kong [16], confirm that when initial data are small smooth perturbations near constant states, blowup in gradient of solutions occurs in finite time if and only if intial data contain any compression in some truly nonlinear characteristic field. A natural puzzle is that: Will this picture keep true for large data problem of physical systems such as compressible Euler equations? One of the key issues is how to find an effective way to obtain sharp enough control on density lower bound. For isentropic flow, we offer a complete picture on the finite time shock formation from smooth initial data away from vacuum, which is consistent with small data theory. For adiabatic flow, we show a striking observation that initial weak compressions do not necessarily develop singularity in finite time, in a sharp contrast to the small data theory. Furthermore, we find the critical strength of nonlinear compression, and prove that if the compression is stronger than this critical value, then singularity develops in finite time, and otherwise there are a class of initial data admitting global smooth solutions with maximum strength of compression equals to this critical value. 
Geng Chen, Ronghua Pan and Shengguo Zhu
math.AP