Grigoryan, A. L. “Estimate of the Remainder of the Best Quadratic Approximation of Differentiable Functions by Polynomials”. Ukrains’kyi Matematychnyi Zhurnal, vol. 56, no. 12, Dec. 2004, pp. 1691-8, https://umj.imath.kiev.ua/index.php/umj/article/view/3876.