Dorogovtsev, A. A., B. Kopytko, V. S. Korolyuk, Y. A. Mitropolskiy, A. M. Samoilenko, G. N. Sytaya, and A. V. Skorokhod. “Mykola Ivanovych Portenko (On His 60th Birthday)”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 2, Feb. 2002, pp. 147-8, https://umj.imath.kiev.ua/index.php/umj/article/view/4050.