Vanishing Uniform Turán Densities: Characterization and Jumps

Source abstract

Uniform Turán density problems ask for the minimum density threshold that guarantees a copy of a fixed 3-uniform hypergraph $F$ in every sufficiently large 3-uniform hypergraph whose edges are uniformly distributed. This line of research was initiated by Erdős and Sós in the 1980s, who introduced the notion of \emph{vertex-set Turán density} $π_{vvv}(F)$. Reiher, Rödl and Schacht later defined two related concepts, namely, the \emph{vertex-pair Turán density} $π_{ve}(F)$ and the \emph{cherry Turán density} $π_{ee}(F)$, which satisfy the chain of inequalities $π_{vvv}(F)\geπ_{ve}(F)\geπ_{ee}(F)$. In this paper, we investigate the problem for their vanishing densities. First, we characterize the 3-uniform hypergraphs $F$ for which $π_{ee}(F)=0$, thereby answering a question posed by Schacht. We show that these are precisely the so-called \emph{max-layered} 3-uniform hypergraphs recently introduced by Lamaison. Furthermore, we answer an analogous question of Lamaison for the \emph{minimum-codegree palette density} $π_{ee}^{\rm pal}$. Together, these results imply that the following are equivalent: $π_{ee}(F)=0$, $π_{ee}^{\rm pal}(F)=0$, and $F$ is a max-layered 3-uniform hypergraph. Let $\star \in \{vvv, ve, ee \}$. We say that zero is a \emph{jump} for $π_{\star}$ if there exists $c>0$ such that no $3$-uniform hypergraph $F$ satisfies $π_{\star}(F)\in(0,c)$. A famous result of Reiher, Rödl and Schacht asserts that zero is a jump for $π_{vvv}$. In contrast, we prove that zero is not a jump for either $π_{ve}$ or $π_{ee}$. This also yields a negative answer to Schacht's well-ordering question for $π_{ve}$. Together with existing results, these findings complete the picture for all three notions of uniform Turán density at zero.