1. Sirotin Ju. N., Shaskolskaja M. P.: Foundations of physics of crystals (in Russian). Nauka, Moscow, 1975.
2. Nye J. F.: Physical properties of crystals. Clarendon Press, Oxford, 1957.
3. Fedorov F. G.: The theory of elastic waves in crystals (in Russian). Nauka, Moscow, 1965.
4. Rychlewski J.: Zur Abschätzung der Anisotropie, ZAMM, 1984, (in print).