1. Seymour, B. R., Mortell, M. P., Non-linear geometrical acoustics in mechanics today, vol.2, (Nemat-Nasser, S., ed.) p. 251. London-New York: Pergamon Press 1975.
2. Varley, E., Cumberbatch, E.: Non-linear, high frequency sound waves. J. Inst. Maths. Appl.2, 133?143 (1966).
3. Courant, R., Hilbert, D.: Methods of mathematical physics New York: Interscience 1962.
4. Davidson, R. C.: Methods in non-linear, plasma theory. New York-London: Academic Press 1972.
5. Thomas, T. Y.: The general theory of compatibility conditions. Int. J. Engng. Sci.4 (3), 207?233 (1966).