BabenkoV. F., and ParfinovychN. V. “On the Order of Relative Approximation of Classes of Differentiable Periodic Functions by Splines”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 2, Feb. 2010, pp. 147–157, https://umj.imath.kiev.ua/index.php/umj/article/view/2851.