ANASTASSIOU, George A., and Merve KESTER. “Quantitative Uniform Approximation by Generalized Discrete Singular Operators”. Studia Universitatis Babeș-Bolyai Mathematica, vol. 60, no. 1, Mar. 2015, pp. 39-60, https://studia.reviste.ubbcluj.ro/index.php/subbmathematica/article/view/5658.