1. N. N. Efimov, T. A. Egorov, A. V. Glushkov et al., in Proceedings of the International Workshop on Astrophysical Aspects of the Most Energetic Cosmic Rays, Kofu 20, 1990.
2. N. Hayashida, K. Honda, M. Honda et al., ICRR Report 324-94-19, Tokyo, 1994.
3. D. J. Bird, S. C. Corbato, H. U. Dai et al., Astrophys. J. 424, 491 (1994).
4. K. Greisen, Phys. Rev. Lett. 2, 748 (1966).
5. G. T. Zatsepin and V. A. Kuz’min, JETP Lett. 4, 78 (1966).