Kofanov, V. A. “The Bojanov – Naidenov Problem for Functions With Nonsymmetric restrictions on the Highest Derivative”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 71, no. 3, Mar. 2019, pp. 368-81, https://umj.imath.kiev.ua/index.php/umj/article/view/1445.