Babenko, V. F., and V. N. Glushko. 1994. “On the Uniqueness of Elements of the Best Approximation and the Best One-Sided Approximation in the Space $L_1$”. Ukrains’kyi Matematychnyi Zhurnal 46 (5): 475–483. https://umj.imath.kiev.ua/index.php/umj/article/view/5711.