Fedorenko, A. S. 1999. “On the Best $m$-Term Trigonometric and Orthogonal Trigonometric Approximations of Functions from the Classes $L^{Ψ}_{β,ρ}$”. Ukrains’kyi Matematychnyi Zhurnal 51 (12): 1719-21. https://umj.imath.kiev.ua/index.php/umj/article/view/4776.