Forbidding $K_{2,t}$ traces in triple systems

TL;DR

This paper investigates the maximum size of 3-uniform hypergraphs avoiding certain trace configurations, specifically $K_{2,t}$ and $C_4$, establishing asymptotic bounds as the number of vertices grows large.

Contribution

It provides new upper bounds on the number of edges in hypergraphs that do not contain $K_{2,t}$ as a trace, including precise asymptotic limits for large $t$ and bounds for $C_4$ traces.

Findings

Asymptotic limit for $K_{2,t}$ traces as $t o \infty$ is $rac{1}{6} t^{3/2} n^{3/2}$.

Upper and lower bounds for the maximum edges avoiding $C_4$ traces are proportional to $n^{3/2}$.

The results extend trace avoidance theory in hypergraph extremal problems.

Abstract

Let $H$ and $F$ be hypergraphs. We say $H$ contains $F$ as a trace if there exists some set $S \subseteq V(H)$ such that $H|_S:=\{E\cap S: E \in E(H)\}$ contains a subhypergraph isomorphic to $F$. In this paper we give an upper bound on the number of edges in a $3$-uniform hypergraph that does not contain $K_{2,t}$ as a trace when $t$ is large. In particular, we show that $ \lim_{t\to \infty}\lim_{n\to \infty} \frac{\mathrm{ex}(n, \mathrm{Tr}_3(K_{2,t}))}{t^{3/2}n^{3/2}} = \frac{1}{6}.$ Moreover, we show $\frac{1}{2} n^{3/2} + o(n^{3/2}) \leq \mathrm{ex}(n, \mathrm{Tr}_3(C_4)) \leq \frac{5}{6} n^{3/2} + o(n^{3/2})$.