Induced Tur\'an numbers

TL;DR

This paper extends Turán-type extremal results to graphs forbidding certain subgraphs both as induced and non-induced, providing new bounds and insights into the structure of such graphs.

Contribution

It proves that classical Turán bounds hold for graphs forbidding a fixed subgraph both as induced and non-induced, and derives bounds on clique counts in these graphs.

Findings

Classical Turán bounds extend to induced and non-induced forbidden subgraphs.

Derived bounds on the number of fixed-size cliques in $K_r$-free graphs with induced $K_{s,t}$-free condition.

Established a new angle for generalizing Turán theory to induced subgraph constraints.

Abstract

The classical K\H{o}v\'ari-S\'os-Tur\'an theorem states that if $G$ is an $n$-vertex graph with no copy of $K_{s,t}$ as a subgraph, then the number of edges in $G$ is at most $O(n^{2-1/s})$. We prove that if one forbids $K_{s,t}$ as an induced/ subgraph, and also forbids any/ fixed graph $H$ as a (not necessarily induced) subgraph, the same asymptotic upper bound still holds, with different constant factors. This introduces a nontrivial angle from which to generalize Tur\'an theory to induced forbidden subgraphs, which this paper explores. Along the way, we derive a nontrivial upper bound on the number of cliques of fixed order in a $K_r$-free graph with no induced copy of $K_{s,t}$. This result is an induced analog of a recent theorem of Alon and Shikhelman and is of independent interest.