Combinatorial results implied by many zero divisors in a group ring

Fedor Petrov

Funkcionalʹnyj analiz i ego priloženiâ, Tome 58 (2024) no. 1, pp. 104-116

Résumé

In a paper of Croot, Lev and Pach and a later paper of Ellenberg and Gijswijt, it was proved that for a group $G=G_0^n$, where $G_0\ne \{1,-1\}^m$ is a fixed finite Abelian group and $n$ is large, any subset $A\subset G$ without 3-progressions (triples $x$, $y$, $z$ of different elements with $xy=z^2$) contains at most $|G|^{1-c}$ elements, where $c>0$ is a constant depending only on $G_0$. This is known to be false when $G$ is, say, a large cyclic group. The aim of this note is to show that the algebraic property corresponding to this difference is the following: in the first case, a group algebra $\mathbb{F}[G]$ over a suitable field $\mathbb{F}$ contains a subspace $X$ with codimension at most $|X|^{1-c}$ such that $X^3=0$. We discuss which bounds are obtained for finite Abelian $p$-groups and for some matrix $p$-groups: the Heisenberg group over $\mathbb{F}_p$ and the unitriangular group over $\mathbb{F}_p$. We also show how the method allows us to generalize the results of [14] and [12].

Détail

Keywords: group ring, zero divisors, $p$-groups.
Mots-clés : arithmtic progressions