Gor'kavyi, V. A., & Nevmerzhitskaya, E. N. (2011). Two-dimensional pseudospherical surfaces with degenerate Bianchi transformation. Ukrains’kyi Matematychnyi Zhurnal, 63(11), 1460-1468. https://umj.imath.kiev.ua/index.php/umj/article/view/2818