BabenkoV. F., and Azar Leis. 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), 1443-51. https://umj.imath.kiev.ua/index.php/umj/article/view/4796.