1. Bowden, R. L., Green berg, W., and Zweifel, P. F.: 1979,J. Math. Phys. 20, 1099
2. Case, K. M.: 1960,Ann. Phys. 9, 1.
3. Chandrasekhar, S.: 1950,Radiative Transfer, Clarendon Press, Oxford.
4. Dunford, N. and Schwartz, J. T.: 1971,Linear Operators, Spectral Operators Interscience, New York.
5. Germogenova, T. A.: 1972, Preprint IPM AN SSSR, No. 21, Moscow (in Russian).