Union-Closed vs Upward-Closed Families of Finite Sets

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 $2\left(\genfrac{}{}{0ex}{}{n}{\lfloor n/2\rfloor }\right)+\left(\genfrac{}{}{0ex}{}{n}{\lfloor n/2\rfloor +1}\right)$ where $|X|=n$.

40 pages