Solving $a\pm b=2c$ in elements of finite sets

Vsevolod F. Lev¹ ; Rom Pinchasi²

¹ Department of Mathematics The University of Haifa at Oranim Tivon 36006, Israel
² Department of Mathematics Technion – Israel Institute of Technology Haifa 32000, Israel

Acta Arithmetica, Tome 163 (2014) no. 2, pp. 127-140.

Résumé

We show that if $A$ and $B$ are finite sets of real numbers, then the number of triples $(a,b,c)\in A\times B\times (A\cup B)$ with $a+b=2c$ is at most $(0.15+o(1))(|A|+|B|)^2$ as $|A|+|B|\to \infty $. As a corollary, if $A$ is antisymmetric (that is, $A\cap (-A)=\emptyset $), then there are at most $(0.3+o(1))|A|^2$ triples $(a,b,c)$ with $a,b,c\in A$ and $a-b=2c$. In the general case where $A$ is not necessarily antisymmetric, we show that the number of triples $(a,b,c)$ with $a,b,c\in A$ and $a-b=2c$ is at most $(0.5+o(1))|A|^2$. These estimates are sharp.

Détail

DOI : 10.4064/aa163-2-3

Keywords: finite sets real numbers number triples times times cup infty corollary antisymmetric cap a emptyset there triples a b general where necessarily antisymmetric number triples a b these estimates sharp