A. A. Shumeiko, “On lower bounds for the approximation of functions by local splines with nonfixed nodes”, Ukr. Mat. Zhurn., vol. 52, no. 1, pp. 134–144, Jan. 2000, Accessed: Apr. 09, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4403