Serdyuk, A. S., and O. I. Stepanets. “Approximation by Fourier Sums and Best Approximations on Classes of Analytic Functions”. Ukrains’kyi Matematychnyi Zhurnal 52, no. 3 (March 25, 2000): 375–395. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4428.