Conjugation groups and structure groups of quandles

Abstract

Quandles are certain algebraic structures showing up in different mathematical contexts. A group $G$ with the conjugation operation forms a quandle, $\mathrm{Conj}(G)$. In the opposite direction, one can construct a group $\mathrm{As}(Q)$ starting from any quandle $Q$. These groups are useful in practice, but hard to compute. We explore the group $\mathrm{As}(\mathrm{Conj}(G))$ for so-called $\overline{C}$-groups $G$. These are groups admitting a presentation with only conjugation and power relations. Symmetric groups $S_n$ are typical examples. We show that for $\overline{C}$-groups, $\mathrm{As}(\mathrm{Conj}(G))$ injects into $G\times {\mathbb{Z}}^m$, where $m$ is the number of conjugacy classes of $G$. From this we deduce information about the torsion, center, and derived group of $\mathrm{As}(\mathrm{Conj}(G))$. As an application, we compute the second quandle homology group of $\mathrm{Conj}(S_n)$ for all $n$, and unveil rich torsion therein.

22 pages. Some references added in v2