Gal, Sorin G. “Degree of Approximation of Continuous Functions by Some Singular Integrals”. Rev. Anal. Numér. Théor. Approx., vol. 27, no. 2, Aug. 1998, pp. 251-6, https://ictp.acad.ro/jnaat/journal/article/view/1998-vol27-no2-art7.