Vakarchuk, S. B., and M. B. Vakarchuk. “$K$-Functionals and Extreme Problems of the Theory of Approximation of the Classes of Analytic Functions in a Circle. II”. Ukrains’kyi Matematychnyi Zhurnal 74, no. 7 (August 9, 2022): 921 - 929. Accessed October 7, 2022. https://umj.imath.kiev.ua/index.php/umj/article/view/6981.