Shumeiko, A. A. “On Lower Bounds for the Approximation of Functions by Local Splines With Nonfixed Nodes”. Ukrains’kyi Matematychnyi Zhurnal 52, no. 1 (January 25, 2000): 134-144. Accessed June 21, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4403.