Andrienko, V. A. “On the Almost-Everywhere Convergence of the Riesz Means of Double Orthogonal Series”. Ukrains’kyi Matematychnyi Zhurnal 51, no. 7 (July 25, 1999): 867–880. Accessed February 17, 2026. https://umj.imath.kiev.ua/index.php/umj/article/view/4676.