Samoilenko , A. M., A. A. Berezovsky, and U. S. Suyarov. 1992. “School-Seminar «Nonlinear Problems of Mathematical Physics and Their Applications» ”. Ukrains’kyi Matematychnyi Zhurnal 44 (5): 720. https://umj.imath.kiev.ua/index.php/umj/article/view/7973.