BerezovskyA. A., KonetI. M., LenyukM. P., SamoilenkoA. M., & TeplinskyY. V. (1997). Ukrainian school-seminar "Nonlinear boundary value problems of mathematical physics and applications". Ukrains’kyi Matematychnyi Zhurnal, 49(4), 613–617. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/5041