Babenko, V. F., and N. V. Parfinovych. “On the Best $L_1$-Approximations of Functional Classes by Splines under Restrictions Imposed on Their Derivatives”. Ukrains’kyi Matematychnyi Zhurnal 51, no. 4 (April 25, 1999): 435–444. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4629.