MelnikYu. I. 1988. “Direct Theorems of Approximation for the Functions Regular in Convex Polygon by Exponential Polynomials in the Integral Metrics ”. Ukrains’kyi Matematychnyi Zhurnal 40 (5), 584-91. https://umj.imath.kiev.ua/index.php/umj/article/view/8806.