Savchuk, V. V. “Best Approximation by Holomorphic Functions. Application to the Best Polynomial Approximation of Classes of Holomorphic Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, no. 8, Aug. 2007, pp. 1047–1067, https://umj.imath.kiev.ua/index.php/umj/article/view/3369.