S. B. Vakarchuk and M. B. . Vakarchuk, “$K$-functionals and extreme problems of the theory of approximation of the classes of analytic functions in a circle. II”, Ukr. Mat. Zhurn., vol. 74, no. 7, pp. 921–929, Aug. 2022, doi: 10.37863/umzh.v74i7.6981.