Stepanets, O. I. “Approximation Characteristics of the Spaces $S_p^{ϕ}$ in Different Metrics”. Ukrains’kyi Matematychnyi Zhurnal, vol. 53, no. 8, Aug. 2001, pp. 1121-46, https://umj.imath.kiev.ua/index.php/umj/article/view/4331.