1. Williams, M.M.R., Proc. Cambridge Phil. Soc. 84, 549 (1978).
2. Lekkerkerker, C.G., Proc. Royal Soc. Edinburgh 83A, 303 (1979).
3. Chandrasekhar, S., Radiative Transfer, Dover, New York, 1960.
4. Maslennikov, M.V., Proc. Steklov Inst. Math. No. 97, Amer. Math. Soc., Providence, 1969.
5. Mee, C.V.M.van der, Math. Centre Tract No. 146, Vrije Universiteit, Amsterdam, 1981.