Codegree-Constrained Tur\'an Problems for $F_5$ and Its Blow-ups

Abstract

Let $F_5$ denote the $3$-uniform hypergraph with edge set $\{abc,abd,cde\}$. For a hypergraph property $\mathcal P$, let $ex_{\mathcal P}(n,F_5)$ be the maximum number of edges in an $n$-vertex $F_5$-free $3$-uniform hypergraph satisfying $\mathcal P$. Let $\Delta_2(H)$ denote the maximum codegree of a pair of vertices in a $3$-uniform hypergraph $H$. We determine the exact value of $ex_{\Delta_2\ge m}(n,F_5)$ when $m$ is bounded away from $n/3$ by a fixed positive proportion. More precisely, for every constant $c\in (1/3,1)$, there exists $n_0$ such that for all $n\ge n_0$ and $m=cn$, \[ ex_{\Delta_2\ge m}(n,F_5) = m\left\lceil\frac{n-m}{2}\right\rceil \left\lfloor\frac{n-m}{2}\right\rfloor. \] The unique extremal hypergraph is the complete tripartite $3$-graph $K^3_{m,\lceil (n-m)/2\rceil,\lfloor (n-m)/2\rfloor}$. We also prove the corresponding exact result for the blow-ups $F_5^t$, together with stability theorems showing that every near-extremal configuration is close to the relevant tripartite construction.