Kopytko, B. I., and M. M. Osypchuk. “Mykola Ivanovych Portenko (on His 80th Birthday) ”. Ukrains’kyi Matematychnyi Zhurnal 75, no. 11 (November 30, 2023): 1446–1449. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/7250.