Ivanova-Karatopraklieva, I. “Infinitesimal Bendings of Rotational Surfaces With Changing Signs Curvature”.
Ann. Sofia Univ. Fac. Math. And Inf.
, vol. 87, Dec. 1995, pp. 179-88, https://admin.uni-sofia.bg/index.php/fmi/article/view/412.