RomanyukV. S. 2001. “Estimates of the Kolmogorov Widths of Classes of Analytic Functions Representable by Cauchy-Type Integrals. I”. Ukrains’kyi Matematychnyi Zhurnal 53 (2), 229-37. https://umj.imath.kiev.ua/index.php/umj/article/view/4238.