1. Baschek, B., Efimov, G.V., Waldenfels, W. von, Wehrse, R., 1997a. Astron. Astrophys. 317, 630.
2. Baschek, B., Grüber, C., Waldenfels, W. von, Wehrse, R., 1997b. Astron. Astrophys. 320, 920.
3. Erdélyi, A., Magnus, W., Oberhettinger, F., Tricomi, F.G., 1953. Higher Transcendental Functions, vol. I. McGraw-Hill, New York.
4. The opacity of expanding media - The effect of spectral lines
5. Learner, R.C.M., 1982, J. Phys. B 15, L891.