Babenko, V. F., and V. N. Glushko. “On the Uniqueness of Elements of the Best Approximation and the Best One-Sided Approximation in the Space $L_1$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 5, May 1994, pp. 475–483, https://umj.imath.kiev.ua/index.php/umj/article/view/5711.