Berezovsky, A. A., Konet, I. M., Lenyuk, M. P., Samoilenko, A. M., & Teplinsky, Y. V. (1997). Ukrainian school-seminar "Nonlinear boundary value problems of mathematical physics and applications". Ukrains’kyi Matematychnyi Zhurnal, 49(4), 613–617. https://umj.imath.kiev.ua/index.php/umj/article/view/5041