An efficient asymmetric removal lemma and its limitations

TL;DR

This paper improves bounds on asymmetric removal lemmas, providing a regularity-free proof of Csaba's theorem and exploring the limitations of $K_3$-abundance in triangle-free tripartite graphs.

Contribution

It offers a regularity-free proof of Csaba's asymmetric triangle removal theorem and investigates the conditions under which certain graphs are $K_3$-abundant, revealing limitations under a number theory conjecture.

Findings

Improved the bound on the number of $C_5$ copies to polynomial in $ ext{varepsilon}$.

Provided a regularity-free proof of Csaba's theorem.

Showed that not all triangle-free tripartite graphs are $K_3$-abundant under a conjecture.

Abstract

The triangle removal states that if $G$ contains $\varepsilon n^2$ edge-disjoint triangles, then $G$ contains $\delta(\varepsilon)n^3$ triangles. Unfortunately, there are no sensible bounds on the order of growth of $\delta(\varepsilon)$, and at any rate, it is known that $\delta(\varepsilon)$ is not polynomial in $\varepsilon$. Csaba recently obtained an asymmetric variant of the triangle removal, stating that if $G$ contains $\varepsilon n^2$ edge-disjoint triangles, then $G$ contains $2^{-\mathrm{poly}(1/\varepsilon)}\cdot n^5$ copies of $C_5$. To this end, he devised a new variant of Szemer\'edi's regularity lemma. We obtain the following results: - We first give a regularity-free proof of Csaba's theorem, which improves the number of copies of $C_5$ to the optimal number $\mathrm{poly}(\varepsilon)\cdot n^5$. - We say that $H$ is $K_3$-abundant if every graph containing $\varepsilon n^2$ edge-disjoint triangles has $\mathrm{poly}(\varepsilon)\cdot n^{|V(H)|}$ copies of $H$. It is easy to see that a $K_3$-abundant graph must be triangle-free and tripartite. Given our first result, it is natural to ask if all triangle-free tripartite graphs are $K_3$-abundant. Our second result is that assuming a well-known conjecture of Ruzsa in additive number theory, the answer to this question is negative. Our proofs use a mix of combinatorial, number-theoretic, probabilistic, and Ramsey-type arguments.