On exactly covering systems of congruences having moduli occurring at most twice Nechemia Burshtein; Johanan Schönheim — 1974 Czechoslovak Mathematical Journal
Old and new necessary and sufficient conditions on ( a i , m i ) in order that n ≡ a i ( mod m i ) be a covering system Štefan Porubský; Johanan Schönheim — 2003 Mathematica Slovaca