Gor’kavyiV. A., and NevmerzhitskayaE. N. “Degenerate Backlund Transformation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 1, Jan. 2016, pp. 38-51, https://umj.imath.kiev.ua/index.php/umj/article/view/1820.