1. N. F. Mott and E. A. Davies,Electron Processes in Non-Crystalline Materials (Clarendon Press, Oxford, 1979).
2. B. I. Shklovskii and A. L. Efros,Electronic Properties of Doped Semiconductors (Nauka, Moscow, 1979) (in Russian).
3. E. Abrahams, P. W. Anderson, D. C. Licciardello, and T. V. Ramakrishnan,Phys. Rev. Lett. 42, 673 (1979).
4. P. Sheng, B. Abeles, M. D. Coutts, and Y. Arie,Adv. Phys. 24, 408 (1975).
5. Yu. G. Morozov, I. G. Naumenko, and V. I. Petinov,Fiz. Nizk. Temp. 2, 987 (1976).