Nilpotent Groups of Class Three and Braces

Abstract

New constructions of braces on finite nilpotent groups are given and hence this leads to new solutions of the Yang--Baxter equation. In particular, it follows that if a group $G$ of odd order is nilpotent of class three, then it is a homomorphic image of the multiplicative group of a finite left brace (i.e.\ an involutive Yang--Baxter group) which also is a nilpotent group of class three. We give necessary and sufficient conditions for an arbitrary group $H$ to be the multiplicative group of a left brace such that $[H,H] \subseteq \operatorname{Soc} (H)$ and $H/[H,H]$ is a standard abelian brace, where $\operatorname{Soc} (H)$ denotes the socle of the brace $H$.