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