Block Designs with Large Transversal-Block Intersections

Abstract

A block design is a set family, whose members are called blocks, where every pair of points in the universe is covered by exactly one block. Erdős asked the following question: is it true that if the universe is of size $n$ and each block is of size $Ω(\sqrt{n})$, then there exists a transversal intersecting every block in $O(1)$ points? We provide a negative answer to this question. Indeed, for arbitrarily large $n$, we construct such a block designs where every transversal intersects some block in $Ω(\frac{\log n}{\log\log n})$ points.