# Some tight lower bounds for Tur\'{a}n problems via constructions of multi-hypergraphs

## TL;DR

This paper establishes tight lower bounds for hypergraph Turán problems by constructing multi-hypergraphs using a modified random algebraic method, clarifying how Turán numbers depend on large parameters of forbidden hypergraphs.

## Contribution

It provides explicit bounds for Turán numbers of complete multipartite and bipartite hypergraphs, revealing their dependence on large parameters, and extends the random algebraic method to multi-hypergraph constructions.

## Key findings

- Determined the asymptotic Turán numbers for complete r-partite r-uniform hypergraphs with large parameters.
- Established the asymptotic Turán numbers for complete bipartite r-uniform hypergraphs with large parameters.
- Confirmed the dependence of Turán numbers on large parameters aligns with classical bounds like Kővári–Sós–Turán.

## Abstract

Recently, several hypergraph Tur\'{a}n problems were solved by the powerful random algebraic method. However, the random algebraic method usually requires some parameters to be very large, hence we are concerned about how these Tur\'{a}n numbers depend on such large parameters of the forbidden hypergraphs. In this paper, we determine the dependence on such specified large constant for several hypergraph Tur\'{a}n problems. More specifically, for complete $r$-partite $r$-uniform hypergraphs, we show that if $s_{r}$ is sufficiently larger than $s_{1},s_{2},\ldots,s_{r-1},$ then $$\textup{ex}_{r}(n,K_{s_{1},s_{2},\ldots,s_{r}}^{(r)})=\Theta(s_{r}^{\frac{1}{s_{1}s_{2}\cdots s_{r-1}}}n^{r-\frac{1}{s_{1}s_{2}\cdots s_{r-1}}}).$$ For complete bipartite $r$-uniform hypergraphs, we prove that if $s$ is sufficiently larger than $t,$ we have $$\textup{ex}_{r}(n,K_{s,t}^{(r)})=\Theta(s^{\frac{1}{t}}n^{r-\frac{1}{t}}).$$ In particular, our results imply that the famous K\H{o}v\'{a}ri--S\'{o}s--Tur\'{a}n's upper bound $\textup{ex}(n,K_{s,t})=O(t^{\frac{1}{s}}n^{2-\frac{1}{s}})$ has the correct dependence on large $t$. The main approach is to construct random multi-hypergraph via a variant of random algebraic method.