1. Gaisser, T.K., to appear in Proc. of Int. Workshop on Observing Ultra High Energy Cosmic Rays from Space and on Earth, Metepec, Mexico, 9-12 Aug 2000, e-print archive: astro-ph/0011525
2. Ambrosio, M. et al., Astrophys. J. 546 (2001) 1038
3. Mannheim, K., to appear Proc. of Int. Symposium on High Energy Gamma-Ray Astronomy, Heidelberg, Germany, 26-30 Jun 2000, e-Print archive: astro-ph/0010353
4. Gaisser, T. K., Halzen, F. & Stanev, T., Phys. Rep., 258 (1995) 173
5. Gaisser, T.K., Nucl. Phys. B (Proc. Suppl.) 48 (1996) 405