AbdullayevF. G., and KüçükaslanM. 2002. “On the Convergence of Fourier Series With Orthogonal Polynomials Inside and on the Closure of a Region”. Ukrains’kyi Matematychnyi Zhurnal 54 (10), 1299-1312. https://umj.imath.kiev.ua/index.php/umj/article/view/4169.