The Turán Density of Tight Cycles in Three-Uniform Hypergraphs

Source abstract

Abstract The Turán density of an $r$-uniform hypergraph ${\mathcal {H}}$, denoted $\pi ({\mathcal {H}})$, is the limit of the maximum density of an $n$-vertex $r$-uniform hypergraph not containing a copy of ${\mathcal {H}}$, as $n \to \infty$. Denote by ${\mathcal {C}}_{\ell }$ the $3$-uniform tight cycle on $\ell$ vertices. Mubayi and Rödl gave an “iterated blow-up” construction showing that the Turán density of ${\mathcal {C}}_{5}$ is at least $2\sqrt {3} - 3 \approx 0.464$, and this bound is conjectured to be tight. Their construction also does not contain ${\mathcal {C}}_{\ell }$ for larger $\ell$ not divisible by $3$, which suggests that it might be the extremal construction for these hypergraphs as well. Here, we determine the Turán density of ${\mathcal {C}}_{\ell }$ for all large $\ell$ not divisible by $3$, showing that indeed $\pi ({\mathcal {C}}_{\ell }) = 2\sqrt {3} - 3$. To our knowledge, this is the first example of a Turán density being determined where the extremal construction is an iterated blow-up construction. A key component in our proof, which may be of independent interest, is a $3$-uniform analogue of the statement “a graph is bipartite if and only if it does not contain an odd cycle”.