Hembars’kaS. B. “Estimates for the Variation of Functions Defined by Double Trigonometric Cosine Series”. Ukrains’kyi Matematychnyi Zhurnal 55, no. 6 (June 25, 2003): 733-749. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3949.