On local Turán density problems of hypergraphs

Abstract: For integers $q\ge p\ge r\ge2$, we say that an $r$-uniform hypergraph $H$ has property $(q,p)$, if for any $q$-vertex subset $Q$ of $V(H)$, there exists a $p$-vertex subset $P$ of $Q$ spanning a clique in $H$. Let $$T_{r}(n,q,p)=\min{ e(H): H\subset \binom{[n]}{r}, H \text{~has property~} (q,p)}$$. The local Tur\'an density about property $(q,p)$ in $r$-uniform hypergraphs is defined as $$t_{r}(q,p)=\lim_{n\to \infty}T_{r}(n,q,p)/\binom{n}{r}$$. Frankl, Huang and R\"odl [J. Comb. Theory, Ser. A, 177 (2021)] showed that $$\lim_{p\to\infty}t_{r}(ap+1,p+1)=\frac{1}{a<sup>{r-1}}$$ for positive integer $a$ and $t_{3}(2p+1,p+1)=\frac{1}{4}$ for all $p\ge 3$ and asked the question that determining the value of $\lim_{p\to\infty}t_{r}(\gamma p+1,p+1)$, where $\gamma\ge 1$ is a real number. Based on the study of hypergraph Tur\'an densities, we determine some exact values of local Tur\'an densities and answer their question partially; in particular, our results imply that the equality in their question about exact values does not hold in general.