Mykola Ivanovych Portenko (On His 60th Birthday) - Dorogovtsev, A. A.; Kopytko, B.I.; Korolyuk, V. S.; Mitropolskiy, Yu. A.; Samoilenko, A. M.; Sytaya, G. N.; Skorokhod, A. V. (Ukrains’kyi Matematychnyi Zhurnal, 2002)