Dorogovtsev, A. A., Kopytko, B., & Osipchuk, M. M. (2017). Mykola Ivanovych Portenko (on his 75th birthday). Ukrains’kyi Matematychnyi Zhurnal, 69(12), 1714-1716. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/1814