CherneіN. I. “Criterion of Stable Convexity of a Region in One-Sheeted Conformal Mappings. I ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 18, no. 1, Jan. 1966, pp. 86-91, https://umj.imath.kiev.ua/index.php/umj/article/view/8655.