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jetpletters.ac.ru
G. T. Zatsepin, V. A. Kuz'min: Upper Limit of the Spectrum of Cosmic Rays. In: Journal of Experimental and Theoretical Physics Letters. 4. Jahrgang, 1966, S.78–80, bibcode:1966JETPL...4...78Z (englisch, jetpletters.ac.ru [PDF]).