Babenko, V. F., & Leis, A. (1998). The best $L_1$-approximations of classes of functions defined by differential operators in terms of generalized splines from these classes.
Ukrains’kyi Matematychnyi Zhurnal,
50(11), 1443-1451.
https://umj.imath.kiev.ua/index.php/umj/article/view/4796