On regularity lemmas and their algorithmic applications

TL;DR

This paper advances the understanding and algorithmic applications of Szemerédi's regularity lemma, providing faster algorithms, improved bounds, and new approximation techniques for related combinatorial problems.

Contribution

It offers new insights into the impact of equitable partitions, improves algorithmic efficiency for subgraph counting, and introduces approximation algorithms for regularity and related problems.

Findings

Equitable partitions have minimal effect on the number of parts.

Polynomial-time approximation algorithms for subgraph counting.

Improved exponential bounds for the permutation regularity lemma.

Abstract

Szemer\'edi's regularity lemma and its variants are some of the most powerful tools in combinatorics. In this paper, we establish several results around the regularity lemma. First, we prove that whether or not we include the condition that the desired vertex partition in the regularity lemma is equitable has a minimal effect on the number of parts of the partition. Second, we use an algorithmic version of the (weak) Frieze--Kannan regularity lemma to give a substantially faster deterministic approximation algorithm for counting subgraphs in a graph. Previously, only an exponential dependence for the running time on the error parameter was known, and we improve it to a polynomial dependence. Third, we revisit the problem of finding an algorithmic regularity lemma, giving approximation algorithms for several co-NP-complete problems. We show how to use the weak Frieze--Kannan regularity lemma to approximate the regularity of a pair of vertex subsets. We also show how to quickly find, for each $\epsilon'>\epsilon$, an $\epsilon'$-regular partition with $k$ parts if there exists an $\epsilon$-regular partition with $k$ parts. Finally, we give a simple proof of the permutation regularity lemma which improves the tower-type bound on the number of parts in the previous proofs to a single exponential bound.