Korneichuk, N. P. “On the Best Approximation of Periodic Functions of Two Variables by Polynomial Splines”. Ukrains’kyi Matematychnyi Zhurnal 52, no. 1 (January 25, 2000): 52–57. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4396.