Tur\'an numbers of extensions

Abstract

The extension of an $r$-uniform hypergraph $G$ is obtained from it by adding for every pair of vertices of $G$, which is not covered by an edge in $G$, an extra edge containing this pair and $r-2$ new vertices. Keevash and Sidorenko~ have previously determined Tur\'an densities of two families of hypergraph extensions. We determine the Tur\'an numbers for these families, using classical stability techniques and new tools introduced in our earlier paper.