1. This lecture is not a complete review of the subject. The literature is quite extensive and several interesting contributions have been omitted.
2. B. Schmidt, J. Fluid Mech. 39, 361 (1969).
3. E.P. Muntz and L.N. Harnett, Phys. Fluids 12, 2027 (1969).
4. D. Gilbarg and D. Paolucci, J. Rat. Mech. Analysis 2, 617 (1953).
5. Two trajectories hit the downstream singular point (from opposite directions), but only one is compatible with the expected monotonic change within a shock wave.