Rivertz, H. J. (2009) “On an invariant on isometric immersions into spaces of constant sectional curvature”, Ukrains’kyi Matematychnyi Zhurnal, 61(12), pp. 1660-1704. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/3129 (Accessed: 24April2024).