Maximal fractional cross-intersecting families

TL;DR

This paper characterizes all maximal fractional cross-intersecting families of subsets of [n] for fractions between 0 and 1, excluding 1/2, extending previous results and answering an open question in the field.

Contribution

It provides a complete characterization of maximal fractional cross-intersecting pairs for fractions between 0 and 1, excluding 1/2, which was previously unresolved.

Findings

Characterization of all maximal fractional cross-intersecting pairs for 0<c/d<1, c/d≠1/2.

Extends previous work on maximal pairs for c/d in {0, 1/2, 1}.

Answers an open question posed by Mathew, Ray, and Srivastava (2019).

Abstract

Given an irreducible fraction $\frac{c}{d} \in [0,1]$, a pair $(\mathcal{A},\mathcal{B})$ is called a $\frac{c}{d}$-cross-intersecting pair of $2^{[n]}$ if $\mathcal{A}, \mathcal{B}$ are two families of subsets of $[n]$ such that for every pair $A \in\mathcal{A}$ and $B\in\mathcal{B}$, $|A \cap B|= \frac{c}{d}|B|$. Mathew, Ray, and Srivastava [{\it\small Fractional cross intersecting families, Graphs and Comb., 2019}] proved that $|\mathcal{A}||\mathcal{B}|\le 2^n$ if $(\mathcal{A}, \mathcal{B})$ is a $\frac{c}{d}$-cross-intersecting pair of $2^{[n]}$ and characterized all the pairs $(\mathcal{A},\mathcal{B})$ with $|\mathcal{A}||\mathcal{B}|=2^n$, such a pair also is called a maximal $\frac cd$-cross-intersecting pair of $2^{[n]}$, when $\frac cd\in\{0,\frac12, 1\}$. In this note, we characterize all the maximal $\frac cd$-cross-intersecting pairs $(\mathcal{A},\mathcal{B})$ when $0<\frac{c}{d}<1$ and $\frac cd\not=\frac 12$, this result answers a question proposed by Mathew, Ray, and Srivastava (2019).