$L^p$-form of the KNRS conjecture

Abstract

The Kohayakawa--Nagle--Rödl--Schacht conjecture predicts that locally dense graphs contain, asymptotically, at least as many homomorphic copies of any fixed graph as the random graph of the same edge density. We prove that every graph with at least one edge satisfies a natural $L^p$ relaxation of this conjecture in the graphon setting. More precisely, let $F$ be a graph with $m>0$ edges, and let $n$ be the number of non-isolated vertices of $F$. If $$ p\ge \binom {n}{2}/m, $$ then for every $ρ$-locally dense graphon $W$, $$ t(F,W^{\circ p})\ge ρ^{pm}. $$ Equivalently, if $$ W_F(\mathbf x)=\prod_{ij\in E(F)}W(x_i,x_j), $$ then $$ \|W_F\|_{L^p}\ge ρ^{e(F)}. $$ The proof is based on a Hölder uniformization over vertex relabellings, in the spirit of Conlon--Lee. We also prove a more general comparison principle with edge-transitive KNRS supergraphs, yielding sharper exponents whenever $F$ embeds into an edge-transitive KNRS graph. Finally, positive-semidefinite methods give theta-subdivision results: Sidorenko-good graphs are closed under arbitrary uniform theta-subdivisions; the non-uniform theta theorem of Im--Li--Liu admits a Sidorenko-good lift, under the same divisibility assumptions, after removing the parity restriction; and uniform theta-subdivisions of KNRS graphs are regular-KNRS.