N. P. Korneichuk, “On the best approximation of periodic functions of two variables by polynomial splines”, Ukr. Mat. Zhurn., vol. 52, no. 1, pp. 52–57, Jan. 2000, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4396