1. The problems presented here do not represent an exhaustive study. Linear piezoelectricity (Voigt [1928]) is a flourishing field. For further study reference may be made to Cady [1946], Dieulesaint and Royer [1980], Mason [1950], Nelson [1979], Ristic [1983], Tiersten [1969], and Narasimhamurty [1981]. Nonlinear piezoelectric and other electromagnetic effects in crystals are dealt with by Maugin [1985] and by Eringen [1980, Chap. 10] see also Toupin [1956].
2. In the sense of Kellog [1929, p. 112].
3. See Nelson [1979, Sect. 11.7].
4. However, such a surface-wave motion may exist in a purely elastic but nonhomogeneous material (Bakirtas and Maugin [1982]), and in elastic ferroelectrics for certain conditions of Bias fields (Maugin [1983]).
5. Eringen [1962, Sect. 115] and [1963].