EmetsO. A. “Extremal Properties of Nondifferentiable Convex Functions on Euclidean Sets of Combinations With Repetitions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 6, June 1994, pp. 680–691, https://umj.imath.kiev.ua/index.php/umj/article/view/5695.