On Mubayi's Conjecture and conditionally intersecting sets

Abstract

Mubayi's Conjecture states that if $\mathcal{F}$ is a family of $k$-sized subsets of $[n] = \{1,\ldots,n\}$ which, for $k \geq d \geq 2$, satisfies $A_1 \cap\cdots\cap A_d \neq \emptyset$ whenever $|A_1 \cup\cdots\cup A_d| \leq 2k$ for all distinct sets $A_1,\ldots,A_d \in\mathcal{F}$, then $|\mathcal{F}|\leq \binom{n-1}{k-1}$, with equality occurring only if $\mathcal{F}$ is the family of all $k$-sized subsets containing some fixed element. This paper proves that Mubayi's Conjecture is true for all families that are invariant with respect to shifting; indeed, these families satisfy a stronger version of Mubayi's Conjecture. Relevant to the conjecture, we prove a fundamental bijective duality between $(i,j)$-unstable families and $(j,i)$-unstable families. Generalising previous intersecting conditions, we introduce the $(d,s,t)$-conditionally intersecting condition for families of sets and prove general results thereon. We conjecture on the size and extremal structures of families $\mathcal{F}\in\binom{[n]}{k}$ that are $(d,2k)$-conditionally intersecting but which are not intersecting, and prove results related to this conjecture. We prove fundamental theorems on two $(d,s)$-conditionally intersecting families that generalise previous intersecting families, and we pose an extension of a previous conjecture by Frankl and Füredi on $(3,2k-1)$-conditionally intersecting families. Finally, we generalise a classical result by Erdős, Ko and Rado by proving tight upper bounds on the size of $(2,s)$-conditionally intersecting families $\mathcal{F}\subseteq 2^{[n]}$ and by characterising the families that attain these bounds. We extend this theorem for certain parametres as well as for sufficiently large families with respect to $(2,s)$-conditionally intersecting families $\mathcal{F}\subseteq 2^{[n]}$ whose members have at most a fixed number $u$ members.