KopytkoB. I., and OsypchukM. M. “Mykola Ivanovych Portenko (on His 80th Birthday) ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 75, no. 11, Nov. 2023, pp. 1446 -49, doi:10.3842/umzh.v75i11.7250.