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arXiv · 1208.5371

Union-Closed vs Upward-Closed Families of Finite Sets

Emanuele Rodaro
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Abstract

A finite family $\mathrsfs{F}$ of subsets of a finite set $X$ is union-closed whenever $f,g\in\mathrsfs{F}$ implies $f\cup g\in\mathrsfs{F}$. These families are well known because of Frankl's conjecture. In this paper we developed further the connection between union-closed families and upward-closed families started in Reimer (2003) using rising operators. With these techniques we are able to obtain tight lower bounds to the average of the length of the elements of $\mathrsfs{F}$ and to prove that the number of joint-irreducible elements of $\mathrsfs{F}$ can not exceed $2{n\choose \lfloor n/2\rfloor}+{n\choose \lfloor n/2\rfloor+1}$ where $|X| = n$.

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Emanuele Rodaro. 2012-10-13. Union-Closed vs Upward-Closed Families of Finite Sets. https://arxiv.org/abs/1208.5371

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