Fedchenko, Yu. S. “Normal and Tangential Geodesic Deformations of the Surfaces of Revolution”.
Ukrains’kyi Matematychnyi Zhurnal 61, no. 10 (October 25, 2009): 1396–1402. Accessed April 14, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/3109.