Plaksa, S. A. “Differentiation of Singular Integrals With Piecewise-Continuous Density and Boundary Values of Derivatives of a Cauchy-Type Integral”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 2, Feb. 2005, pp. 222–229, https://umj.imath.kiev.ua/index.php/umj/article/view/3589.