Kasperski, Andrzej. “Approximation of Elements of the Spaces X^1_ϕ and X_ϕ by Nonlinear, Singular Kernels”. Annales Mathematicae Silesianae 6 (September 30, 1992): 21–29. Accessed May 6, 2025. https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/14264.