Ivanova-Karatopraklieva, Ivanka. “Infinitesimal Bendings of Rotational Surfaces With Changing Signs Curvature”. Ann. Sofia Univ. Fac. Math. and Inf. 87 (December 12, 1995): 179–188. Accessed November 21, 2024. https://ftl5.uni-sofia.bg/index.php/fmi/article/view/412.