Cross-intersecting families and primitivity of symmetric systems

Jun Wang, Huajun Zhang

Let $X$ be a finite set and $\mathfrak{p}\subseteq 2^X$, the power set of $X$, satisfying three conditions: (a) $\mathfrak{p}$ is an ideal in $2^X$, that is, if $A\in \mathfrak{p}$ an…