Chong Shangguan (上官冲), On the sparse hypergraph problem of Brown, Erdős and Sós

For fixed integers $r\ge 3, e\ge 3$, and $v\ge r+1$, let $f_r(n,v,e)$ denote the maximum number of edges in an $n$-vertex $r$-uniform hypergraph in which the union of arbitrary $e$ distinct edges contains at least $v+1$ vertices. In 1973, Brown, Erdős and Sós initiated the study of the function $f_r(n,v,e)$ and they proved that $\Omega(n^{\frac{er-v}{e-1}})=f_r(n,v,e)=O(n^{\lceil\frac{er-v}{e-1}\rceil})$. We will survey the state-of-art results about the study of $f_r(n,er-(e-1)k+1,e)$ and $f_r(n,er-(e-1)k,e)$, where $r>k\ge 2$ and $e\ge 3$. Although these two functions have been extensively studied, many interesting questions remain open.