# Polylogarithmic bounds in the nilpotent Freiman theorem

## TL;DR

This paper establishes polylogarithmic bounds for approximate subgroups in nilpotent groups, generalizing abelian results and improving bounds in Freiman-type theorems for residually nilpotent and linear groups.

## Contribution

It introduces polylogarithmic bounds in the nilpotent Freiman theorem, extending abelian results to nilpotent groups and improving existing bounds.

## Key findings

- Bounds are polylogarithmic in the size of the approximate subgroup.
- Existence of a nilprogression within a controlled power of the approximate subgroup.
- Improved rank bounds in Freiman-type theorems for residually nilpotent and linear groups.

## Abstract

We show that if $A$ is a finite $K$-approximate subgroup of an $s$-step nilpotent group then there is a finite normal subgroup $H\subset A^{K^{O_s(1)}}$ modulo which $A^{O_s(\log^{O_s(1)}K)}$ contains a nilprogression of rank at most $O_s(\log^{O_s(1)}K)$ and size at least $\exp(-O_s(\log^{O_s(1)}K))|A|$. This partially generalises the close-to-optimal bounds obtained in the abelian case by Sanders, and improves the bounds and simplifies the exposition of an earlier result of the author. Combined with results of Breuillard-Green, Breuillard-Green-Tao, Gill-Helfgott-Pyber-Szab\'o, and the author, this leads to improved rank bounds in Freiman-type theorems in residually nilpotent groups and certain linear groups of bounded degree.