Positive co-degree Tur\'an number for $C_5$ and $C_5^{-}$

Abstract

The \emph{minimum positive co-degree} $\delta^{+}_{r-1}(H)$ of a non-empty $r$-graph $H$ 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 $k$ hyperedges of $H$. For any $r$-graph $F$, the \emph{positive degree Tur\'an number} $\mathrm{co}^{+}\mathrm{ex}(n,F)$ is defined as the maximum value of $\delta^{+}_{r-1}(H)$ over all $n$-vertex $F$-free non-empty $r$-graphs $H$. In this paper, we determine the positive degree Tur\'an number for $C_5$ and $C_5^{-}$.