Cross-intersecting sub-families of hereditary families

Peter Borg

Families ${\mathcal{A}}_1,{\mathcal{A}}_2,...,{\mathcal{A}}_k$ of sets are said to be \emph{cross-intersecting} if for any $i$ and $j$ in $\{1,2,...,k\}$ with $i\ne j$, any set i…