Vyhovs'ka, I. Yu., and Yu. B. Zelinskii. “Criterion for Convexity of a Domain of a Euclidean Space”. Ukrains’kyi Matematychnyi Zhurnal, vol. 60, no. 5, May 2008, pp. 709–711, https://umj.imath.kiev.ua/index.php/umj/article/view/3189.