
Péter L. Erdős

Dániel Gerbner

Nathan Lemons

Dhruv Mubayi

Cory Palmer

Balázs Patkós
Keywords:
extremal set theory, Sperner, intersecting
Abstract
The two part Sperner theorem of Katona and Kleitman states that if $X$ is an $n$element set with partition $X_1 \cup X_2$, and $\mathcal{F}$ is a family of subsets of $X$ such that no two sets $A, B \in \mathcal{F}$ satisfy $A \subset B$ (or $B \subset A$) and $A \cap X_i=B\cap X_i$ for some $i$, then $\mathcal{F} \le {n \choose \lfloor n/2\rfloor}$. We consider variations of this problem by replacing the Sperner property with the intersection property and considering families that satisfy various combinations of these properties on one or both parts $X_1$, $X_2$. Along the way, we prove the following new result which may be of independent interest: let $\mathcal{F},\mathcal{G}$ be intersecting families of subsets of an $n$element set that are additionally crossSperner, meaning that if $A \in\mathcal{F}$ and $B \in \mathcal{G}$, then $A \not\subset B$ and $B \not\subset A$. Then $\mathcal{F} +\mathcal{G} \le 2^{n1}$ and there are exponentially many examples showing that this bound is tight.