Kolesnik, L. I., and Andrashkо M. I. “Inverse Theorems of Approximation on the Average in Regions With Angles”. Ukrains’kyi Matematychnyi Zhurnal 23, no. 1 (December 25, 1970): 97–104. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/8487.