KASPERSKI, Andrzej. Approximation of elements of the spaces X^1_ϕ and X_ϕ by nonlinear, singular kernels. Annales Mathematicae Silesianae, [S. l.], v. 6, p. 21–29, 1992. Disponível em: https://journals.us.edu.pl/index.php/AMSIL/article/view/14264. Acesso em: 5 dec. 2025.