DorogovtsevA. A., KopytkoB.I., and OsipchukM. M. “Mykola Ivanovych Portenko (on His 75th Birthday)”. Ukrains’kyi Matematychnyi Zhurnal 69, no. 12 (December 25, 2017): 1714-1716. Accessed November 23, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/1814.