Kasperski, Andrzej. 1992. “Approximation of Elements of the Spaces X^1_ϕ and X_ϕ by Nonlinear, Singular Kernels”. Annales Mathematicae Silesianae 6 (September), 21-29. https://www.journals.us.edu.pl/index.php/AMSIL/article/view/14264.