Stasyuk, S. A. 2010. “Best $m$-Term Trigonometric Approximation for the Classes $B^r_{p,θ}$ of Functions of Low Smoothness”. Ukrains’kyi Matematychnyi Zhurnal 62 (1): 104–111. https://umj.imath.kiev.ua/index.php/umj/article/view/2846.