1. Courant, R. and K. O. Friedrichs, Supersonic Flow and Shock Waves, Interscience, New York, 1948.
2. Mises, R. von, Mathematical Theory of Compressible Fluid Flow, Academic Press, New York, 1958.
3. Martin, M. H., Canadian J. Math.5 (1953) 37.
4. Ludford, G. S. S. and M. H. Martin, Commun. pure appl. Math.7 (1954) 45.