KotsurI. M., and KotsurM. F. “On the Radii of Convexity and Starlikeness for Some Special Classes of Analytic Functions in a Disk”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 7, July 1996, pp. 954-8, https://umj.imath.kiev.ua/index.php/umj/article/view/5268.