Rivertz, H. J. “On an Invariant on Isometric Immersions into Spaces of Constant Sectional Curvature”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 61, no. 12, Dec. 2009, pp. 1660-04,
https://umj.imath.kiev.ua/index.php/umj/article/view/3129
.