Shkapa, V. V. “Approximating Characteristics of the Classes $L_{β,p}^{ψ}$ of Periodic Functions in the Space $L_q$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 67, no. 8, Aug. 2015, pp. 1139-50, http://umj.imath.kiev.ua/index.php/umj/article/view/2052.