A Solution to a Question of Bouchard

We prove an intersection lemma for finite indexed families of finite sets, allowing repetitions. If $c,m\ge1$ and $m\ge\ |\bigcup_{i\in[m]}C_i|+c$, then there is a nonempty set of indices $K\subseteq[m]$ such that $|\bigcap_{i\in K}C_i|=|K|-c$. By taking complements in a suitably enlarged universe, we obtain an affirmative answer to Question 2.2 in Bouchard's paper "Conjectures on union-closed families of sets" (2024). Consequently, $UC_x$ holds for every union-closed family with a nonempty $n$-element universe and every integer $1\le x\le\lfloor n/2\rfloor+1$.

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Publication Details

Journal
Zenodo (CERN European Organization for Nuclear Research)
Published
2026-10-27
DOI
https://doi.org/10.5281/zenodo.23691321
Primary Topic
Advanced Combinatorial Mathematics
Type
preprint
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preprint

A Solution to a Question of Bouchard

Alessio Faretra
Zenodo (CERN European Organization for Nuclear Research)
Advanced Combinatorial Mathematics
preprint

A Solution to a Question of Bouchard

Alessio Faretra
preprint en

Abstract

We prove an intersection lemma for finite indexed families of finite sets, allowing repetitions. If $c,m\ge1$ and $m\ge\ |\bigcup_{i\in[m]}C_i|+c$, then there is a nonempty set of indices $K\subseteq[m]$ such that $|\bigcap_{i\in K}C_i|=|K|-c$. By taking complements in a suitably enlarged universe, we obtain an affirmative answer to Question 2.2 in Bouchard's paper "Conjectures on union-closed families of sets" (2024). Consequently, $UC_x$ holds for every union-closed family with a nonempty $n$-element universe and every integer $1\le x\le\lfloor n/2\rfloor+1$.

Zenodo (CERN European Organization for Nuclear Research)
Advanced Combinatorial Mathematics
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A Solution to a Question of Bouchard — Alessio Faretra · Zenodo (CERN European Organization for Nuclear Research) (2026) | TGRS Research Map | TGRS