Stepanets, A. I., and N. L. Pachulia. “On Behaviour of deviations’ Group on Sets of $(\psi,\beta)$-Differentiable Functions”. Ukrains’kyi Matematychnyi Zhurnal, vol. 40, no. 1, Dec. 1987, pp. 101-5, https://umj.imath.kiev.ua/index.php/umj/article/view/9063.