English

Constructing subsets of a given packing index in Abelian groups

Group Theory 2009-01-12 v1 Combinatorics

Abstract

By definition, the sharp packing index \indP(A)\ind_P^\sharp(A) of a subset AA of an abelian group GG is the smallest cardinal κ\kappa such that for any subset BGB\subset G of size Bκ|B|\ge\kappa the family {b+A:bB}\{b+A:b\in B\} is not disjoint. We prove that an infinite Abelian group GG contains a subset AA with given index \indP(A)=κ\ind_P^\sharp(A)=\kappa if and only if one of the following conditions holds: (1) 2κG+2\le \kappa\le|G|^+ and k{3,4}k\notin \{3,4\}; (2) κ=3\kappa=3 and GG is not isomorphic to iIZ3\oplus_{i\in I} \mathbb{Z}_3; (3) κ=4\kappa=4 and GG is not isomorphic to iIZ2\oplus_{i\in I} \mathbb{Z}_2 or to Z4(iIZ2)\mathbb{Z}_4\oplus(\oplus_{i\in I} \mathbb{Z}_2).

Keywords

Cite

@article{arxiv.0901.1151,
  title  = {Constructing subsets of a given packing index in Abelian groups},
  author = {N. Lyaskovska},
  journal= {arXiv preprint arXiv:0901.1151},
  year   = {2009}
}

Comments

9 pages

R2 v1 2026-06-21T11:58:55.973Z