Prykarpatsky, A. K., and V. G. Samoilenko. “Structure of Binary Darboux-Type Transformations for Hermitian Adjoint Differential Operators”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 2, Feb. 2004, pp. 271-5, https://umj.imath.kiev.ua/index.php/umj/article/view/3751.