Konohrai, A. F., & Stasyuk, S. A. (2008). Best M-term trigonometric approximations of the classes of periodic functions of many variables in the space Lq. Ukrains’kyi Matematychnyi Zhurnal, 60(9), 1196 – 1214. https://umj.imath.kiev.ua/index.php/umj/article/view/3237