FedorenkoA. S., and FedorenkoO. “The Best $m$-Term Trigonometric Approximations of the Classes $L_{\beta ,p}^\Psi$ in Uniform Metric”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 1, Jan. 2004, pp. 129-32, https://umj.imath.kiev.ua/index.php/umj/article/view/3735.