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FORCING QUASIRANDOMNESS WITH TRIANGLES

Published online by Cambridge University Press:  02 April 2019

CHRISTIAN REIHER
Affiliation:
Fachbereich Mathematik, Universität Hamburg, Hamburg, Germany; Christian.Reiher@uni-hamburg.de, schacht@math.uni-hamburg.de
MATHIAS SCHACHT
Affiliation:
Fachbereich Mathematik, Universität Hamburg, Hamburg, Germany; Christian.Reiher@uni-hamburg.de, schacht@math.uni-hamburg.de

Abstract

We study forcing pairs for quasirandom graphs. Chung, Graham, and Wilson initiated the study of families ${\mathcal{F}}$ of graphs with the property that if a large graph $G$ has approximately homomorphism density $p^{e(F)}$ for some fixed $p\in (0,1]$ for every $F\in {\mathcal{F}}$, then $G$ is quasirandom with density $p$. Such families ${\mathcal{F}}$ are said to be forcing. Several forcing families were found over the last three decades and characterizing all bipartite graphs $F$ such that $(K_{2},F)$ is a forcing pair is a well-known open problem in the area of quasirandom graphs, which is closely related to Sidorenko’s conjecture. In fact, most of the known forcing families involve bipartite graphs only.

We consider forcing pairs containing the triangle $K_{3}$. In particular, we show that if $(K_{2},F)$ is a forcing pair, then so is $(K_{3},F^{\rhd })$, where $F^{\rhd }$ is obtained from $F$ by replacing every edge of $F$ by a triangle (each of which introduces a new vertex). For the proof we first show that $(K_{3},C_{4}^{\rhd })$ is a forcing pair, which strengthens related results of Simonovits and Sós and of Conlon et al.

MSC classification

Type
Research Article
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
© The Author(s) 2019

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