Positive co-degree density of hypergraphs

Abstract: The \emph{minimum positive co-degree} of a non-empty $r$-graph ${H}$, denoted $\delta_{r-1}⁺⁽ {H})$, is the maximum $k$ such that if $S$ is an $(r-1)$-set contained in a hyperedge of ${H}$, then $S$ is contained in at least $k$ distinct hyperedges of ${H}$. Given an $r$-graph ${F}$, we introduce the \emph{positive co-degree Tur\'an number} $\mathrm{co^+ex}(n, {F})$ as the maximum positive co-degree $\delta_{r-1}<sup>+(H)$ over all $n$-vertex $r$-graphs $H$ that do not contain $F$ as a subhypergraph. In this paper we concentrate on the behavior of $\mathrm{co^+ex}(n, {F})$ for $3$-graphs $F$. In particular, we determine asymptotics and bounds for several well-known concrete $3$-graphs $F$ (e.g.\ $K_4<sup>-$ and the Fano plane). We also show that, for $r$-graphs, the limit [ \gamma^+(F) := \lim_{n \rightarrow \infty} \frac{\mathrm{co^+ex}(n, {F})}{n} ] exists, and ``jumps'' from $0$ to $1/r$, i.e., it never takes on values in the interval $(0,1/r)$. Moreover, we characterize which $r$-graphs $F$ have $\gamma<sup>+(F)=0$. Our motivation comes primarily from the study of (ordinary) co-degree Tur\'an numbers where a number of results have been proved that inspire our results.