@article{10_4064_aa_59_1_59_70,
     author = {R. Simpson and Doron Zeilberger},
     title = {Necessary conditions for distinct covering systems with square-free moduli},
     journal = {Acta Arithmetica},
     pages = {59--70},
     publisher = {mathdoc},
     volume = {59},
     number = {1},
     year = {1991},
     doi = {10.4064/aa-59-1-59-70},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/aa-59-1-59-70/}
}