Dorogovtsev, A. A., B. Kopytko, and M. M. Osipchuk. “Mykola Ivanovych Portenko (on His 75th Birthday)”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 69, no. 12, Dec. 2017, pp. 1714-6, https://umj.imath.kiev.ua/index.php/umj/article/view/1814.