Large Cliques and Clique Spectral Radius in the Erd\H{o}s--S\'{o}s Problem

Abstract

For graphs $H$ and $F$, let $ex(n,H,F)$ be the maximum number of copies of $H$ in an $n$-vertex $F$-free graph. We study this problem when $H$ is a clique and $F=T_t$which is a fixed tree on $t$ vertices. The Erd\H{o}s--S\'{o}s conjecture concerns the value of $ex(n,K_2, T_t)$. Gerbner and Palmer proposed a more general conjecture: if $n=\alpha(t-1)+\beta$ and $0\le\beta\le t-2$, then the graph $\alpha K_{t-1}\sqcup K_\beta$ maximizes the number of $r$-cliques among all $n$-vertex $T_t$-free graphs for every $3\le r\le t-2$. We show that this conjecture holds for $T_t$ having at least $t-r$ leaves with a common parent, which contains the star case as a special case and recovers the sharp clique-counting result conjectured by Gan, Loh and Sudakov and proved by Chase and Chao and Dong. We also study the clique-spectral analogue. Under the same leaf-bunch condition, every $T_t$-free graph $G$ satisfies $\rho_r(G)\le\binom{t-2}{r-1}$, with equality, for $n\ge t-1$, if and only if $K_{t-1}$ is a component of $G$. Furthermore, we prove the conjecture for $r=t-d$ whenever $d\ge2$ and $t\ge d^2-d+3$, while the case $d=1$ is determined exactly for every $t$. For $d\ge2$ and $t\ge d^2-d+3$, every $T_t$-free graph $G$ satisfies $\rho_{t-d}(G)\le\rho_{t-d}(K_{t-1})$, with equality characterized by the presence of a $K_{t-1}$-component. Our method is designed for relatively large cliques. In the leaf-poor case, after deleting edges that lie in no $(t-d)$-clique, we study the intersection relation among $(t-d)$-cliques and show that its equivalence classes induce the nontrivial clique-supported components; furthermore, we show each non-trivial component has at most $t-1$\) vertices. In the complementary leaf-rich case, a leaf-bunch criterion reduces the clique-counting problem to the sharp bounded-maximum-degree clique theorem.