Venn diagrams as forbidden hypergraph traces

Abstract

We study the maximum size of a set system that contains no $k$-Venn diagram, denoted by $V{D}_k$, as a trace. For every fixed $k\ge 3$, we prove ${\text{ex}}_{tr}(n,V{D}_k)=O_k(n^{2^k-2k+1})$, improving the direct Sauer-Shelah bound $O_k(n^{2^k-1})$. In particular, for $k=4$ the exponent decreases from $15$ to $9$. The proof starts from the theorem of Keevash, Leader, Long and Wagner for $V{D}_3$ and uses induction on $k$ in which two new Venn regions are forced for free at each added edge. We also record lower-bound constructions for Venn diagrams in fixed uniformity, explicit bounds for the $4$-uniform $3$-Venn problem, and a fixed uniformity trace result for the loose triangle.