Research notes on every problem, and a library of the papers behind them. Built from the open erdos repository.
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Necessary conditions for distinct covering systems with square-free moduli
R. J. Simpson and Doron Zeilberger, Necessary conditions for distinct covering systems with square-free moduli, Acta Arithmetica LIX (1991), no. 1, 59–70, doi:10.4064/aa-59-1-59-70.
The inspected journal PDF is simpson_zeilberger_1991_squarefree_distinct_covering_systems.pdf. A Markdown reading copy sits beside the PDF. The official Acta Arithmetica record confirms the bibliographic record. The file's text layer carries no copyright or license line; the publisher's record labels the PDF download "Pobierz zgodnie z CC-BY", which the English site renders "Free download under CC-BY license", naming no version or license URL (https://www.impan.pl/get/doi/10.4064/aa-59-1-59-70, read 2026-10-02).
This source bears directly on Problem 7 and is an earlier necessary-condition result related to the later Balister et al. square-free obstruction.