English

Combinatorial properties of MAD families

Logic 2026-01-14 v1 General Topology

Abstract

We study some strong combinatorial properties of MAD\textsf{MAD} families. An ideal I\mathcal{I} is Shelah-Stepr\={a}ns if for every set X[ω]<ωX\subseteq{\left[ \omega\right]}^{<\omega} there is an element of I\mathcal{I} that either intersects every set in XX or contains infinitely many members of it. We prove that a Borel ideal is Shelah-Stepr\={a}ns if and only if it is Kat\v{e}tov above the ideal fin×fin\textsf{fin}\times\textsf{fin}. We prove that Shelah-Stepr\={a}ns MAD\textsf{MAD} families have strong indestructibility properties (in particular, they are both Cohen and random indestructible). We also consider some other strong combinatorial properties of MAD\textsf{MAD} families. Finally, it is proved that it is consistent to have non(M)=1\mathrm{non}(\mathcal{M}) = {\aleph}_{1} and no Shelah-Stepr\={a}ns families of size 1{\aleph}_{1}.

Keywords

Cite

@article{arxiv.2206.14936,
  title  = {Combinatorial properties of MAD families},
  author = {Jörg Brendle and Osvaldo Guzmán and Michael Hrušák and Dilip Raghavan},
  journal= {arXiv preprint arXiv:2206.14936},
  year   = {2026}
}

Comments

43 pages. Submitted. arXiv admin note: text overlap with arXiv:1810.09680