Semi-hyperbolic patches of solutions to the two-dimensional nonlinear wave system for Chaplygin gases

Semi-hyperbolic patches are the regions in which one family out of two nonlinear families of characteristics starts on sonic curves and ends on transonic shock waves. This type of region appears frequently in the two-dimensional Riemann problem for the Euler equations and its simplified models and a few other situations. We construct a semi-hyperbolic patch of solution to the two-dimensional nonlinear wave system with Chaplygin gas equation of state by approaching the problem as a Goursat-type boundary value problem which has a sonic curve as the degenerate boundary.