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Cardinality of subsets of the residue group with nonunit differences of elements

  • Pavel V. Roldugin EMAIL logo
Published/Copyright: June 26, 2017

Abstract

The paper is concerned with the subsets I ⊂ {0, ..., d – 1} for which gcd(nm, d) ≠ 1 for any n, mI. Such subsets are called sets of nontrivial differences. Let d > 1 and d1 be the least prime divisor of d. We prove that the largest cardinality of a set of nontrivial differences is d/d1. Sets of nontrivial differences in which not all differences of elements are multiples of the same prime factor d are called nonelementary. Let t be the number of prime factors of d. We show that there are no nonelementary sets for t ⩽ 2. It is shown that a minimal nonelementary set may have arbitrary order in the interval 3,t¯. The largest cardinality of nonelementary sets is estimated from below and above.


Originally published in Diskretnaya Matematika (2016) 28, №3, 111–125 (in Russian).


References

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Received: 2016-2-17
Published Online: 2017-6-26
Published in Print: 2017-6-27

© 2017 Walter de Gruyter GmbH, Berlin/Boston

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