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Discrete spheres and arithmetic progressions in product sets

Volume 178 / 2017

Dmitrii Zhelezov Acta Arithmetica 178 (2017), 235-248 MSC: Primary 11B25. DOI: 10.4064/aa8332-11-2016 Published online: 18 April 2017


We prove that if $B$ is a set of $N$ positive integers such that $B\cdot B$ contains an arithmetic progression of length $M$, then for some absolute $C \gt 0$, $$ \pi(M) + C \frac {M^{2/3}}{\log^2 M} \leq N, $$ where $\pi$ is the prime counting function. This improves on previously known bounds of the form $N = \Omega(\pi(M))$ and gives a bound which is sharp up to the second order term, as Pách and Sándor gave an example for which $$ N \lt \pi(M)+ O\biggl(\frac {M^{2/3}}{\log^2 M} \biggr). $$ The main new tool is a reduction of the original problem to the question of approximate additive decomposition of the $3$-sphere in $\mathbb{F}_3^n$ which is the set of 0-1 vectors with exactly three non-zero coordinates. Namely, we prove that such a set cannot have an additive basis of order two of size less than $c n^2$ with absolute constant $c \gt 0$.


  • Dmitrii ZhelezovDepartment of Mathematical Sciences
    Chalmers University of Technology and University of Gothenburg
    41296 Göteborg, Sweden

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