MotornayaO. V. “On the Asymptotic Estimates of the Best Approximations of Differentiable Functions by Algebraić Polynomials in the Space $L_1$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 45, no. 6, June 1993, pp. 859–862, https://umj.imath.kiev.ua/index.php/umj/article/view/5874.