ABSTRACT
The rays are the bicharacteristic curves of the original quasilinear system and the propagation of the discontinuity is governed by a set of ordinary differential equations along the rays. However, characteristic shocks and intermediate discontinuities are very special cases of shock waves. It would be very interesting to have a ray theory which applies to shock waves of general type as well. In the case of plane weak shocks propagating into a uniform medium there is a mathematically valid theory based on the concept of shock-fitting which dates back to Chandrasekhar and Landau. This theory has been extended to cover non planar geometry by Whitham and to more general situations (non-uniform background state).
