Kadets V. М., and Popov М. М. “The Lyapunov Theorem on Convexity and Its Use for Sign-Embeddings ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 44, no. 9, Oct. 1992, pp. 1192-00, https://umj.imath.kiev.ua/index.php/umj/article/view/8169.