BabenkoV. F., and GorbenkoM. E. 2000. “On the Uniqueness of an Element of the Best $L_1$-Approximation for Functions With Values in a Banach Space”. Ukrains’kyi Matematychnyi Zhurnal 52 (1), 30-34. https://umj.imath.kiev.ua/index.php/umj/article/view/4393.