RomanyukV. S. “Estimates of the Kolmogorov Widths of Classes of Analytic Functions Representable by Cauchy-Type Integrals. I”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 2, Feb. 2001, pp. 229-37, https://umj.imath.kiev.ua/index.php/umj/article/view/4238.