Gor’kavyi, V. A., and E. N. Nevmerzhitskaya. “Two-Dimensional Pseudospherical Surfaces With Degenerate Bianchi Transformation”. Ukrains’kyi Matematychnyi Zhurnal 63, no. 11 (November 25, 2011): 1460-1468. Accessed February 8, 2023. https://umj.imath.kiev.ua/index.php/umj/article/view/2818.