The regularity of sonic curves for the two-dimensional Riemann problems   of the nonlinear wave system of Chaplygin gas
  We study the regularity of sonic curves to a two-dimensional Riemann problem for the nonlinear wave system of Chaplygin gas, which is an essential step for the global existence of solutions to the two-dimensional Riemann problems. As a result, we establish the global existence of uniformly smooth solutions in the semi-hyperbolic patches up to the sonic boundary, where the degeneracy of hyperbolicity occurs. Furthermore, we show the C^1 regularity of sonic curves. 
Qin Wang and Kyungwoo Song
math.AP