KolomiyetsV. G., MitropolskiyY. A., and SamoilenkoA. M. “The Conference ‘Nonlinear Problems of Differential Equations and Mathematical Physics. The Second Bogolyubov readings’”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 4, Apr. 1994, p. 471, https://umj.imath.kiev.ua/index.php/umj/article/view/5751.