Savchuk, V. V., A. M. Samoilenko, I. V. Sokolenko, and O. I. Stepanets. “International Conference ‘Mathematical Analysis and Differential Equations and Applications’”. Ukrains’kyi Matematychnyi Zhurnal 59, no. 3 (March 25, 2007): 431. Accessed April 11, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/3317.