Fushchich, V. I., and V. I. Chopik. 1993. “Symmetry and Non-Lie Reduction of the Nonlinear Schrödinger Equation”. Ukrains’kyi Matematychnyi Zhurnal 45 (4): 539–551. https://umj.imath.kiev.ua/index.php/umj/article/view/5841.