1. The most important data are: AGASA Collaboration: M Takedaet al, Phys. Rev. Lett. 81, 1163 (1998); astro-ph/9807193 HiRes Collaboration: T Abu-Zayyadet al, astro-ph/0208301
2. A M Hillas,Ann. Rev. Astron. Astrophys. 22, 425 (1984)
3. K Greisen,Phys. Rev. Lett. 16, 748 (1966)
4. G T Zatsepin and V A Kuzmin,JETP Lett. 4, 78 (1966);Pisma Zh. Eksp. Teor. Fiz. 4, 114 (1966)
5. D S Gorbunov, P G Tinyakov, I I Tkachev and S V Troitsky,Astrophys. J. 577, L93 (2002); astro-ph/0204360