Zaderei, P. V., and ’ B. A. Smal. “On the Convergence of Fourier Series in the Space $L_1$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 5, May 2002, pp. 639-46, https://umj.imath.kiev.ua/index.php/umj/article/view/4102.