L. I. Kolesnik and Andrashkо M. I., “Inverse theorems of approximation on the average in regions with angles”, Ukr. Mat. Zhurn., vol. 23, no. 1, pp. 97–104, Dec. 1970, Accessed: Feb. 17, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/8487