Rivertz, H. J. “On an Invariant on Isometric Immersions into Spaces of Constant Sectional Curvature”. Ukrains’kyi Matematychnyi Zhurnal 61, no. 12 (December 25, 2009): 1660-1704. Accessed March 29, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3129.