Masal’tsevL. A. “On Isometric Immersion of Three-Dimensional Geometries $SL_2$, $Nil$ and $Sol$ into a Four-Dimensional Space of Constant Curvature”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 3, Mar. 2005, pp. 421–426, https://umj.imath.kiev.ua/index.php/umj/article/view/3609.