Dzyubenko, H. A., and V. D. Zalizko. “Coconvex Approximation of Functions With More Than One Inflection Point”. Ukrains’kyi Matematychnyi Zhurnal, vol. 56, no. 3, Mar. 2004, pp. 352-65, https://umj.imath.kiev.ua/index.php/umj/article/view/3759.