1. Ambartsumian, V. A.: 1943,Dokl. Akad. Nauk, SSSR 38, 229.
2. Bellman, R. Kalaba, R., and Ueno, S.: 1962.The Rand Corporation, RM 2269-ARPA.
3. Chandrasekhar, S.: 1960,Radiative Transfer, Dover, New York (reprint).
4. Chandrasekhar, S.: 1958,Proc. Nat. Acad. Sci. 44, 933.
5. Matsumoto, M.: 1967,Publ. Astron. Soc. Japan 19(3), 454.