GogoladzeL., and TsagareishviliV. “Convergence of Fourier Series of Functions $\text{Lip} 1$ With respect to General Orthonormal Systems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 69, no. 4, Apr. 2017, pp. 466-77, https://umj.imath.kiev.ua/index.php/umj/article/view/1710.