Asymptotics of the Brown--Erdős--Sós problem at integer exponents

Abstract

The Brown--Erdős--Sós problem is a fundamental problem in sparse hypergraph Turán theory. For integers $r,k\ge 2$ and $s\ge r$, the problem asks for the maximum number $f^{(r)}(n;s,k)$ of edges in an $n$-vertex $r$-uniform hypergraph containing no $k$ distinct edges spanning at most $s$ vertices. In 1971, Brown, Erdős, and Sós proved that $f^{(r)}(n;(r-t)k+t,k)=\Theta (n^t)$ for all $r>t\ge 2$ and $k\ge 2$. However, the existence and the value of the leading coefficient have remained largely open. We determine the coefficient $\pi (r,t,k):={\lim }_{n\to \infty }{n}^{-t}{f}^{(r)}(n;(r-t)k+t,k)$ for every such $r,t,k$, except when $(r,t)=(3,2)$ and $k\ge 4$ is even. In particular, \[ π(r,t,k) = \begin{cases} \frac{2}{t!\bigl(2\binom{r}{t}-1\bigr)}, & \text{if $k$ is odd},\\ \frac{1}{t!\binom{r}{t}}, & \text{if $k$ is even and $r\ge 4$}. \end{cases} \] Surprisingly, for $r\ge 4$, the limit $\pi (r,t,k)$ depends on $k$ only through its parity, not its value. For the remaining case, we prove that $\pi (3,2,k)>1/6$ for all even $k\ge 4$, showing that the natural packing construction is never asymptotically optimal. As an application, we extend a connection of Bennett, Cushman, and Dudek to arbitrary uniformity to resolve several cases of the Erdős--Gyárfás--Shelah generalized Ramsey problem.

arXiv comment22 pages, 1 figure