“…In [6] Calvaruso and Kowalski calculate Ricci operators for left-invariant Lorentz metrics on three-dimensional Lie groups, assuming they are not symmetric (see also previous curvature calculations in [5,7,19]). If the metric on U \S were symmetric, then the covariant derivative of the curvature would vanish on all of U , which would imply U locally symmetric, hence locally homogeneous; therefore, we need consider only nonsymmetric left-invariant metrics here.…”