BerezovskyA. A., LenyukM. P., and SamoilenkoA. M. “School-Seminar ‘Nonlinear Boundary-Value Problems in Mathematical Physics and Their applications’”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 6, June 1996, pp. 863-5, https://umj.imath.kiev.ua/index.php/umj/article/view/5289.