Bondarenko, A. V., and J. Gilewicz. “Negative Result in Pointwise 3-Convex Polynomial Approximation”. Ukrains’kyi Matematychnyi Zhurnal 61, no. 4 (April 25, 2009): 563-567. Accessed February 5, 2023. https://umj.imath.kiev.ua/index.php/umj/article/view/3040.