1. F. Erdogan, Crack-Propagation Theories, Chapter 5 in Fracture Vol. II., Liebowitz ed., Academic Press, New York and London, (1968) p. 497.
2. F. Kerkhof, Habilitationsschrift, Karlsruhe (1965).
3. Z. P. Chang, Third-Order Elastic Constants of NaCI and KC1 Single Crystals, The Physical Review, 140 (1965) A1788.
4. M. Schinker, unpublished results.
5. N. G. Basov and O. N. Krokhin, Zh. Eksperim i teor. Fiz., 46 (1964) 171.