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2019 Bivariate fluctuations for the number of arithmetic progressions in random sets
Yacine Barhoumi-Andréani, Christoph Koch, Hong Liu
Electron. J. Probab. 24: 1-32 (2019). DOI: 10.1214/19-EJP391

## Abstract

We study arithmetic progressions $\{a,a+b,a+2b,\dots ,a+(\ell -1) b\}$, with $\ell \ge 3$, in random subsets of the initial segment of natural numbers $[n]:=\{1,2,\dots , n\}$. Given $p\in [0,1]$ we denote by $[n]_{p}$ the random subset of $[n]$ which includes every number with probability $p$, independently of one another. The focus lies on sparse random subsets, i.e. when $p=p(n)=o(1)$ as $n\to +\infty$. Let $X_{\ell }$ denote the number of distinct arithmetic progressions of length $\ell$ which are contained in $[n]_{p}$. We determine the limiting distribution for $X_{\ell }$ not only for fixed $\ell \ge 3$ but also when $\ell =\ell (n)\to +\infty$ with $\ell =o(\log n)$. The main result concerns the joint distribution of the pair $(X_{\ell },X_{\ell '})$, $\ell >\ell '$, for which we prove a bivariate central limit theorem for a wide range of $p$. Interestingly, the question of whether the limiting distribution is trivial, degenerate, or non-trivial is characterised by the asymptotic behaviour (as $n\to +\infty$) of the threshold function $\psi _{\ell }=\psi _{\ell }(n):=np^{\ell -1}\ell$. The proofs are based on the method of moments and combinatorial arguments, such as an algorithmic enumeration of collections of arithmetic progressions.

## Citation

Yacine Barhoumi-Andréani. Christoph Koch. Hong Liu. "Bivariate fluctuations for the number of arithmetic progressions in random sets." Electron. J. Probab. 24 1 - 32, 2019. https://doi.org/10.1214/19-EJP391

## Information

Received: 2 April 2019; Accepted: 7 November 2019; Published: 2019
First available in Project Euclid: 28 December 2019

zbMATH: 07149385
MathSciNet: MR4049081
Digital Object Identifier: 10.1214/19-EJP391

Subjects:
Primary: 05C80, 11B25, 60C05, 60F05

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Vol.24 • 2019 