S. Vakarchuk, V. Zabutna, and M. Vakarchuk, “Averaged characteristics of smoothness in $L_2$ and estimations for the widths values of function classes”, Ukr. Mat. Zhurn., vol. 77, no. 2, pp. 83–106, Apr. 2025, doi: 10.3842/umzh.v77i2.8794.