Dzyubenko, H. A., and V. D. Zalizko. “Pointwise Estimates for the Coconvex Approximation of Differentiable Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 1, Jan. 2005, pp. 47–59, https://umj.imath.kiev.ua/index.php/umj/article/view/3573.