UsenkoE. G. “Criteria for the Coincidence of the Kernel of a Function With the Kernels of Its Riesz and Abel Integral Means”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 12, Dec. 1998, pp. 1712–1714, https://umj.imath.kiev.ua/index.php/umj/article/view/4792.