LasuriyaR. A., and StepanetsO. I. “Strong Summability of Orthogonal Expansions of Summable Functions. I”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 2, Feb. 1996, pp. 260-77, https://umj.imath.kiev.ua/index.php/umj/article/view/5353.