Dovgoshei , A. A. “On Integral Representations in the Hardi Classes and on the Best Approximations in Some Functional Spaces”. Ukrains’kyi Matematychnyi Zhurnal, vol. 43, no. 3, Feb. 1991, pp. 342-7, https://umj.imath.kiev.ua/index.php/umj/article/view/9613.