Ku Yong Ha | Searcharxiv

arXiv2022

Left invariant Lorentzian metrics and curvatures on non-unimodular Lie groups of dimension three

For each connected and simply connected three-dimensional non-unimodular Lie group, we classify the left invariant Lorentzian metrics up to automorphism, and study the extent to which curvature can be altered by a change of metric. Thereby we obtain the Ricci operator, the scalar curvature, and the sectional curvatures as functions of left invariant Lorentzian metrics on the three-dimensional non-unimodular Lie groups. Our study is a continuation and extension of the previous studies done in \cite{HL2009_MN} for Riemannian metrics on three-dimensional Lie groups and in \cite{BC} for Lorentzian metrics on three-dimensional unimodular Lie groups.

math.DG↗

arXiv2014

The $R_\infty$ property for crystallographic group of Sol

There are 9 kinds of crystallographic groups $Π$ of Sol. For any automorphism $φ$ on $Π$, we study the Reidemeister number $R(φ)$. Using the averaging formula for the Reidemeister numbers, we prove that most of the crystallographic groups of Sol have the $R_\infty$ property.

math.AT↗