Lasuriya, R. A. “Asymptotics of Approximation of ψ-Differentiable Functions of Many Variables”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 60, no. 8, Aug. 2008, pp. 1051–1057, https://umj.imath.kiev.ua/index.php/umj/article/view/3223.