V. V. Savchuk, A. M. Samoilenko, I. V. Sokolenko, and O. I. Stepanets, “International Conference ‘Mathematical Analysis and Differential Equations and Applications’”, Ukr. Mat. Zhurn., vol. 59, no. 3, p. 431, Mar. 2007, Accessed: Feb. 17, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3317