Induced rational exponents and bipartite subgraphs in $K_{s,s}$-free graphs

Abstract

In this paper, we study a general phenomenon that many extremal results for bipartite graphs can be transferred to the induced setting when the host graph is $K_{s,s}$-free. As manifestations of this phenomenon, we prove that every rational $\frac{a}{b}\in (1,2),a,b\in {\mathbb{N}}_{+}$, can be achieved as Turán exponent of a family of at most $2^a$ induced forbidden bipartite graphs, extending a result of Bukh and Conlon [JEMS 2018]. Our forbidden family is a subfamily of theirs which is substantially smaller. A key ingredient, which is yet another instance of this phenomenon, is supersaturation results for induced trees and cycles in $K_{s,s}$-free graphs. We also provide new evidence to a recent conjecture of Hunter, Milojević, Sudakov, and Tomon [JCTB 2025] by proving optimal bounds for the maximum size of $K_{s,s}$-free graphs without an induced copy of theta graphs or prism graphs, whose Turán exponents were determined by Conlon [BLMS 2019] and by Gao, Janzer, Liu, and Xu [IJM 2025+].

21 pages, 3 figures