Algebraic characterization of reversibility in the quaternionic Möbius group

Abstract

An element of a group is called \emph{reversible} if it is conjugate to its inverse. While reversibility in the quaternionic Möbius group $\mathrm{PSL}(2,\mathbb{H})$ has traditionally been studied using geometric and dynamical methods, we develop a purely algebraic approach. We obtain an explicit, computable criterion for the reversibility of a quaternionic Möbius transformation, expressed solely in terms of the entries of a matrix representative. More precisely, we prove that \[ [A]\in \mathrm{PSL}(2,\mathbb{H}) \text{ is reversible} \quad \Longleftrightarrow \quad β_A^{2}=δ_A^{2}, \] where ${\beta }_A$ and ${\delta }_A$ are real conjugacy invariants associated with a lift $A\in \mathrm{SL}(2,\mathbb{H})$. Furthermore, we give a complete characterization of reversing symmetries of reversible elements in $\mathrm{SL}(2,\mathbb{H})$ and $\mathrm{PSL}(2,\mathbb{H})$.

Final version, 10 pages. To appear in Czechoslovak Mathematical Journal. Substantially revised, with improved exposition and refinements to the title, abstract, and main results