Conjectured classification of Erdős-deep triples of arithmetic progressions

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Let Zn\mathbb{Z}_n be the cyclic group of integers modulo nn. An Erdős-deep family is a family whose distance multiset has multiplicities precisely 1,2,…,k−11,2,\dots,k-1 for some integer kk. Let three arithmetic progressions in Zn\mathbb{Z}_n have lengths k1≥k2≥k3k_1\geq k_2\geq k_3. Classification conjecture. An Erdős-deep family of three arithmetic progressions of these lengths exists if and only if either

(k1,k2,k3)∈{(4,4,3),(6,3,3)},(k_1,k_2,k_3)\in\{(4,4,3),(6,3,3)\},

with each of these two length triples occurring for infinitely many values of nn, or

(k1,k2,k3)∈{(6,5,3),(6,6,4),(6,6,6),(7,7,3),(9,4,3),(8,7,4),(8,8,5),(10,6,4),(12,4,4),(13,5,3),(13,7,4),(16,5,4),(21,6,4)},(k_1,k_2,k_3)\in\{(6,5,3),(6,6,4),(6,6,6),(7,7,3),(9,4,3),(8,7,4),(8,8,5),(10,6,4),(12,4,4),(13,5,3),(13,7,4),(16,5,4),(21,6,4)\},

with each of these length triples occurring for a finite number of values of nn. The conjecture would complete the classification of Erdős-deep triples of arithmetic progressions; the paper reports that it is based on a computer search of small parameters, while the cases of two progressions have already been classified.

References

Primary source

Peter J Dukes and Tao Gaede, “Families of modular arithmetic progressions with an interval of distance multiplicities”, arXiv:2208.05527 (2022).

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