VakarchukS. B. and VakarchukM. B. (2022) “$K$-functionals and extreme problems of the theory of approximation for classes of analytic functions in the circle. I”, *Ukrains’kyi Matematychnyi Zhurnal*, 74(4), pp. 469 - 485. doi: 10.37863/umzh.v74i4.6980.