On Hypergraph Lagrangians and Frankl-F\"uredi's Conjecture

Abstract

Frankl and F\"uredi conjectured in 1989 that the maximum Lagrangian, denoted by $\lambda_r(m)$, among all $r$-uniform hypergraphs of fixed size $m$ is achieved by the minimum hypergraph $C_{r,m}$ under the colexicographic order. We say $m$ in {\em principal domain} if there exists an integer $t$ such that ${t-1\choose r}\leq m\leq {t\choose r}-{t-2\choose r-2}$. If $m$ is in the principal domain, then Frankl-F\"uredi's conjecture has a very simple expression: $$\lambda_r(m)=\frac{1}{(t-1)^r}{t-1\choose r}.$$ Many previous results are focusing on $r=3$. For $r\geq 4$, Tyomkyn in 2017 proved that Frankl-F\"{u}redi's conjecture holds whenever ${t-1\choose r} \leq m \leq {t\choose r} -{t-2\choose r-2}- \delta_rt^{r-2}$ for a constant $\delta_r>0$. In this paper, we improve Tyomkyn's result by showing Frankl-F\"{u}redi's conjecture holds whenever ${t-1\choose r} \leq m \leq {t\choose r} -{t-2\choose r-2}- \delta_r't^{r-\frac{7}{3}}$ for a constant $\delta_r'>0$.