Leiko, S. G., and Yu. S. Fedchenko. “Infinitesimal Rotary Deformations of Surfaces and Their Application to the Theory of Elastic Shells”. Ukrains’kyi Matematychnyi Zhurnal, vol. 55, no. 12, Dec. 2003, pp. 1697-03, https://umj.imath.kiev.ua/index.php/umj/article/view/4031.