[1] Burshtein N.: On natural exactly covering systems of congruences having moduli occurring at most twice. Journal of Number Theory (To appear). MR 0412088 | Zbl 0334.10004

[2] Erdös P.: On a problem concerning congruence systems. Mat. Lapok 3 (1952), 122-128. MR 0070651

[3] Erdös P.: Proceedings of the 1963 Number Theory Conference. University of Colorado. Proposed problem no. 28.

[4] Krukenberg C. E.: Covering sets of the integers. University of Illinois at Urbana-Champaign, 1971 (Ph. D. Thesis). MR 2620587

[5] Porubský Š.: Generalization of some results for exactly covering systems. Mat. Časopis 22 (1972), 208-214. MR 0308021

[6] Porubský Š.: Natural exactly covering systems of congruences. Czech. Math. J. 24 (99) (1974), No 4. MR 0354520

[7] Schönheim J., N. Burshtein: On a conjecture concerning exactly covering systems of congruences. Israel Journal of Math. 8 (1970), 28-29. DOI 10.1007/BF02771546 | MR 0258724

[8] Znám Š.: On exactly covering systems of arithmetic sequences. Math. Ann. 180 (1969), 227-232. DOI 10.1007/BF01350740 | MR 0242760

[9] Znám Š.: On the relation between exactly covering systems and rooted trees. Unpublished.