Masal’tsev, L. 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 57, no. 3 (March 25, 2005): 421–426. Accessed May 26, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3609.