Vakarchuk, S. B., and M. B. Vakarchuk. “$K$-Functionals and Extreme Problems of the Theory of Approximation for Classes of Analytic Functions in the Circle. I”. Ukrains’kyi Matematychnyi Zhurnal 74, no. 4 (May 31, 2022): 469 - 485. Accessed June 28, 2022. https://umj.imath.kiev.ua/index.php/umj/article/view/6980.