1. Ambartsumian, V. A.: 1943,Dokl. Akad. Nauk, SSSR 38, 229.
2. Bellman, R., Kalaba, R., and Lockett, A.: 1966,Numerical Inversion of Laplace Transform, Elsevier New York.
3. Bellman, R., Kalaba, R., and Ueno, S.: 1962,Icarus 1, 191.
4. Biswas, G. and Karanjai, S.: 1989,Astrophys. Space Sci. (submitted).
5. Chandrasekhar, S.: 1960,Radiative Transfer, Dover, New York.