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Solution of the minimum modulus problem for covering systems. (English) Zbl 1344.11015
A covering system (CS) of congruences is a finite collection of residue classes $$a_i\pmod{n_i}$$, $$i=1,\dots,k$$, such that every integer belong to at least one of them. If all the moduli are distinct the covering system is called distinct (author denotes them also simply as covering systems without warning). Immediately after introducing this notion by P. Erdős in the thirties of the last century Erdős formulated several striking problems connected with this notion (see the reviewer’s booklet [“Results and problems on covering systems of residue classes.” Mitt. Math. Semin. Gießen 150 (1981; Zbl 0479.10032)] for more details). Two of them were: (1) minimum modulus problem asking whether there exists a distinct CS the least modulus of which exceeds a given integer; (2 with J. L.Selfridge) odd moduli problem asking if there exists a distinct CS with all moduli odd.
The author of the paper under review proves that the first problem has a negative answer: The least modulus of a distinct CS is at most $$10^{16}$$. The most exciting ingredient of this proof is the Lovász local lemma. (The best known example supporting an affirmative answer to Erdős’ original question is due to P. P. Nielsen [J. Number Theory 129, No. 3, 640–666 (2009; Zbl 1234.11011)] giving an example of a distinct CS with the least modulus 40).
In connection with the second Erdős problem the author notes that his result “immediately implies that any (distinct) CS contains a modulus divisible by one of an initial segment of primes”. In this connection note that M. Billik and H. M. Edgar [Math. Mag. 46, 265–270 (1973; Zbl 0275.10004)] proved that a negative answer to problem (1) implies that any distinct CS with the maximal modulus has an even modulus.

##### MSC:
 11B25 Arithmetic progressions 05B40 Combinatorial aspects of packing and covering 05C70 Edge subsets with special properties (factorization, matching, partitioning, covering and packing, etc.) 11A07 Congruences; primitive roots; residue systems
PARI/GP
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