1. Landau, L. D., and Rumer, G., Phys. Z. Sowjetunion, 11, 18 (1937).
2. For example, Ziman, J. M., Electrons and Phonons (Oxf. Univ. Press).
3. Pomeranchuk, I., J. Phys. U.S.S.R., 4, 259 (1941); 6, 237 (1942).
4. Orbach, R., Ph D. thesis, Univ. California (1956).