PichugovS. A. “Approximation of Compressions of Periodic Functions in the Space $L_p,\; P < 1$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 47, no. 12, Dec. 1995, pp. 1708-11, https://umj.imath.kiev.ua/index.php/umj/article/view/5566.