1. BednarekBignami, G.F. et al., 1981, Astron. Astrophys. 93, 71
2. W., Protheroe, R.J., 1999, Mon. Not. R. Astron. Soc. 310, 577
3. Bloemen, H. et al., 1999, Astro. Lett. and Communications 39, 681
4. Bowyer, S., Leinert, Ch., eds., 1990, The Galactic and Extragalactic Background Radiation, Cambridge University Press, Cambridge
5. Calzetti, D., Livio, M., Madau, P., eds., 1995, Extragalactic Background Radiation, Cambridge University Press, Cambridge