Vakarchuk, S. B., and M. Sh. Shabozov. 1996. “Quasiwidths and Optimization of Methods of Mixed Approximation of Multidimensional Singular Integrals With Kernels of Hilbert Type”. Ukrains’kyi Matematychnyi Zhurnal 48 (6): 753-70. https://umj.imath.kiev.ua/index.php/umj/article/view/5276.