Nikitin, A. G., and R. O. Popovich. “Group Classification of Nonlinear Schrödinger Equations”. Ukrains’kyi Matematychnyi Zhurnal, vol. 53, no. 8, Aug. 2001, pp. 1053-60, https://umj.imath.kiev.ua/index.php/umj/article/view/4326.