On the generalized Turán number of the complete bipartite graph $K_{3,b+1}$

Abstract

For graphs $F$ and $H$, let $\mathrm{ex}(n,H,F)$ denote the maximum number of copies of $H$ in an $n$-vertex $F$-free graph. Very recently, Janzer, Longbrake, and Yepremyan proved that for $3<a\leq b$ and sufficiently large $t$, \begin{equation*} \mathrm{ex}(n,K_{a,b},K_{3,t})=Θ_{a,b,t}(n^3). \end{equation*} Later, Hou, Hu, and Wang made this threshold explicit by showing that the conclusion holds for all $t\geq 2\max\{3,\lceil b/2\rceil\}+1$. In particular, for every even $b\geq 6$, this matches the necessary threshold $t=b+1$. In this paper, we resolve the remaining case where $b$ is odd. More precisely, we prove that for all fixed integers $b\geq 5$ and $3<a\leq b$, \begin{equation*} \mathrm{ex}(n,K_{a,b},K_{3,b+1})=Θ_{a,b}(n^3). \end{equation*} Our construction uses a finite-field point set in $\mathrm{PG}(5,q)$ together with an orthogonal polarity. The key new ingredient is the polynomial splitting lemma due to Andrade, Bary-Soroker, and Rudnick, which produces many planes whose intersections with the point set and their polar planes both have size $b$. This gives a $K_{3,b+1}$-free incidence graph while preserving $Ω_{a,b}(n^3)$ copies of $K_{a,b}$.