CherneіN. I. “Some Theorems on Stable Convexity of Closed Curves under One-Sheeted Mappings”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 23, no. 2, Feb. 1971, pp. 276-80, https://umj.imath.kiev.ua/index.php/umj/article/view/8515.