KorolyukV. S., PortenkoN. I., and SamoilenkoA. M. “Anatolii Volodymyrovych Skorokhod”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 63, no. 6, June 2011, pp. 859 -864, https://umj.imath.kiev.ua/index.php/umj/article/view/2770.